Diablo

Diablo is a male-type demon who initially appears as an upper demon (Arc Demon) with the alias Primordial Black.

After being summoned by Rimuru during the Falmuth Kingdom War, he receives a name, evolves into a Demon Peer, and later awakens as a Devil Lord, one of the Twelve Guardian Lords, with the title Demon Lord (Magin King).

He is infamous as one of the strongest demons in existence, rivaling Guy Crimson in status among the Primordial Demons.

Despite his overwhelming power, he is strangely uninterested in personal strength itself and is instead obsessively devoted to serving Rimuru.

His personality is cunning, calculating, and apparently cold, yet he is utterly loyal, polite to almost everyone, and extremely dangerous when crossed.

He speaks in a formal manner to all, and his signature laugh is “Kufufufufu.”

The origin of his name is the Lamborghini supercar Lamborghini Diablo, itself derived from the Italian word for demon, diavolo.

In adaptations and spin-offs, he is voiced by Takahiro Sakurai.

Diablo’s combat power and abilities escalate dramatically as the story progresses.

By volume 23 of the novels, he has become one of the absolute top-tier beings in the world.

Species / Race Progression: Demon → Upper Demon (Arc Demon) → Demon Peer → Devil Lord (Awakened Demon King–class demon)

Lineage: One of the Seven Primordial Demons, representing Black

Title(s):

Primordial Black

Demon Lord (Magin King) – as one of Rimuru’s Twelve Guardian Lords

Blessing: Blessing of Rimuru Tempest

EP (Existence Points): ~6,666,666 → later ~40,000,000 (comparable to Guy Crimson)

Magical Specializations: Dark magic, elemental magic

Ultimate Skill:

Lord of Temptation (Azazel) – evolved from his Unique Skill Tempter (Otosu-mono)

Functions include: accelerated thinking, universal perception, Demon Lord’s Haki (overwhelming aura), space-time manipulation → later full space-time dominion, multidimensional barriers, all-things analysis, punishment control, charm control, and a personal World of Temptation

Former Unique Skills (both eventually integrated into Azazel):

Tempter (Otosu-mono): mental domination, charm, forced recruitment of those who submit

Great Sage (Motomu-mono): similar to Rimuru’s Daikenja (Great Sage), with accelerated thinking, chant annulment, universal analysis, and law manipulation

Resistances:

Physical attacks nullified

Status ailments nullified

Mental attacks nullified

Natural phenomena nullified

Resistance to holy and demonic attacks

After awakening as a Devil Lord, his magicule quantity becomes comparable to Rimuru’s when Rimuru existed as a spiritual slime body.

This puts Diablo at a level where, under certain conditions and if Rimuru holds back, he might even surpass Rimuru in direct combat potential.

Diablo is extremely cunning, highly intelligent, and frighteningly calm, yet also whimsical and capricious.

He likes to smile pleasantly, but his expression almost always looks sinister or predatory to others.

He is very polite and uses respectful language with almost everyone, regardless of status.

However, when truly enraged, his tone becomes razor-sharp and his face turns completely blank, exuding an eerie, lifeless menace.

Unlike the typical demon, who is obsessed with personal strength and fighting, Diablo does not care about being the strongest for its own sake.

His core obsession is Rimuru: serving Rimuru, helping Rimuru’s plans, and eliminating Rimuru’s enemies.

He considers being useful to Rimuru the highest form of happiness and life’s purpose.

If someone opposes or insults Rimuru, Diablo will classify them as a target for elimination and acts without hesitation.

He constantly praises Rimuru to anyone who will listen, to the point of exhausting Rimuru and even making Guy Crimson uncomfortable.

If he is excluded from important events or missions involving Rimuru, he becomes genuinely heartbroken, treating it as if the world has ended.

Diablo has a long-running rivalry with Shion over the position of Rimuru’s secretary.

They often compete in childish ways, yet they also resonate strongly in their shared devotion to Rimuru and sometimes team up to punish those rude to him.

He treats the Primordial Demon trio (Testarossa, Ultima, and Carrera) as near-equals and is one of the few who himself exercises some self-restraint around them.

However, toward lower-ranking demons and other subordinates, he can be extremely harsh, almost casually brutal, though he does keep Rimuru’s long-term interests in mind and avoids wasting valuable assets without purpose.

He deeply appreciates those who admire or revere Rimuru, regarding them as promising or “with potential.”

Conversely, those who disrespect Rimuru are marked for “correction” or outright deletion.

Notably, Diablo once pushed himself into literal spiritual exhaustion just to teach Shion how to brew proper black tea, apparently out of pure goodwill.

Shion in turn often calls him “tea lackey” or “tea flunky,” meaning both “a small fry who just flatters Rimuru” and “a guy whose only brain cell is about serving tea,” but Diablo does not mind; serving Rimuru tea is an honor in his view.

He is known for being verbally sharp and great at arguments, to the point that many acquaintances find him a hassle to deal with; he has made more than a few girls cry through sheer verbal dominance.

Among demons, he is regarded as eccentric and troublesome, but also terrifyingly competent.

Diablo takes a refined, almost aristocratic butler-like male humanoid form.

He is extremely handsome and looks almost human, aside from his demonic traits.

He wears a formal black butler suit and carries himself like an impeccable servant.

His hair is black with striking red and gold streaks running through it, adding to his flamboyant yet refined appearance.

His eyes are golden with red pupils, and the whites of his eyes are completely black, giving him an unsettling but mesmerizing gaze.

He can manifest bat-like demon wings from his back, which he normally keeps hidden; these can be used both for flight and for defense, as seen when he used them as a shield against flames in an animated side story.

As a high-level spiritual being, Diablo technically has no fixed biological sex and is genderless in essence.

However, he prefers a male form and presentation, and the novels confirm that he can also take on a female form if he chooses, similar to Guy Crimson.

In the original web version of the story, Diablo was described with red hair like Guy, because the “Primordial Demon” color concept had not been fully codified and many demons’ colors overlapped.

The later book version clarified his iconic black hair with red and gold highlights to clearly mark him as the Primordial Black.

For a long time, Rimuru and his subordinates did not know Diablo’s true identity.

In reality, he is one of the Seven Primordial Demons, the oldest and most powerful demons, and is specifically the Primordial Black, the archetype of all black demons.

Among demons, he is one of the very top even within the Primordial group, alongside Primordial Red (Rouge), who later becomes Guy Crimson.

The two once fought in the unfettered environment of the underworld—with no restrictions—over who was the strongest, ending in a draw.

Unlike other Primordials, Diablo rarely creates direct subordinates, making his bloodline extremely rare and somewhat mysterious.

The few demons he does take under his wing tend to be oddballs: curious, selfish, and deviating from typical demon instincts.

He once roamed the world freely without a physical body (no permanent “incarnation”), disdaining the idea of taking followers or ruling over a territory.

In his view, “Tools that aren’t useful are unnecessary, and strength for its own sake is boring.” This kept him solitary for ages.

King Gazel Dwargo knows of a strange legend: a mysterious, nameless demon, with no territory or known servants, who appears across world history and vanishes without a trace—this is Diablo.

In the anime, Razen is shown to be unsettled by Diablo’s eye color, implying that some concrete descriptions of him survived in certain records.

Volume 12 of the novels is the first time his identity as Primordial Black is openly recognized by Rimuru and the others.

Diablo himself never made a big deal of it, simply because he did not consider it important enough to mention.

His few subordinates include Venom, a demon who was once an otherworlder and repeatedly challenged Diablo despite a huge power gap, earning Diablo’s fondness and becoming his direct subordinate.

Another is Beretta, who was originally one of Diablo’s rare direct descendants before being summoned by Rimuru and given a new body.

When Diablo later tried to recruit other Primordials as “helpers” so he could delegate tasks and spend more time near Rimuru, he sought out Testarossa, Ultima, and Carrera.

He also bullied or “reeducated” countless lesser demons in the process, and, in at least one external work, casually annihilated nearly a thousand High Demons who might otherwise have been summoned instead of him.

Diablo’s entire character is defined by his relationship with Rimuru Tempest.

He first becomes interested in Rimuru indirectly, through his encounter with Shizue Izawa and her anomalous mask, and then later, when Rimuru inherits Shizu’s will and appearance.

After Shizu passes away and Rimuru adopts her form and ideals, Diablo secretly observes Rimuru from the shadows.

By the time Rimuru becomes a Demon Lord, Diablo has already spent pages worth of internal monologue praising Rimuru’s charm in spin-off material.

When Rimuru performs the massive demon summoning during the Falmuth war, it is technically a summoning circle intended for High Demons only.

Diablo fakes being a High Demon to slip through the constraints and appear, though many High Demons who might have answered the call are quietly massacred by him beforehand.

Upon being summoned, Diablo assists Rimuru in annihilating the Falmuth Kingdom forces and carries out the post-battle cleanup.

He shows his loyalty immediately by offering up two of his own High Demon followers as sacrifices to provide enough magicules for Rimuru’s mass resurrection spell—and those two cheerfully accept the honor, as revealed in the spin-off The Slime Diaries.

During the celebration after Rimuru’s Demon Lord awakening, Diablo goes to meet his summoner.

However, Rimuru—who had summoned him half-asleep and under extreme fatigue—has completely forgotten him, forcing Diablo to introduce himself from scratch.

When Rimuru looks at him and says, “Who are you again?”, Diablo’s heart nearly shatters.

Rimuru then offhandedly says, “You’re still here? You can go home now if you want,” which Diablo interprets as being abandoned after being unable to return, causing his heart-core to completely fracture; he nearly bursts into tears.

Once Rimuru recognizes the misunderstanding, he formally takes Diablo as a subordinate and gives him a name.

The naming consumes half of Rimuru’s newly increased magicule pool, showcasing Diablo’s terrifying existence value.

After being named Diablo, he evolves from an Arc Demon into a Demon Peer.

Later, when Rimuru transfers 100,000 souls to Diablo as a reward and tool for future battles, Diablo awakens as a Devil Lord (Devil Road), his ultimate skill evolving into Lord of Temptation (Azazel).

Diablo then becomes one of Rimuru’s “Three Peaks”—the trio of strongest subordinates—arguably the strongest among them.

He also becomes one of the Twelve Guardian Lords, bearing the title Demon Lord (Magin King).

Unique Skills and Ultimate Ability

Tempter (Otosu-mono)

Tempter is a Unique Skill that allows Diablo to dominate those who submit to him.

It grants him overwhelming control over a target’s life and death and compels their obedience through fear.

Its main functions include:

Thought Control: Direct manipulation of the target’s mind and decisions

Charm: Ensnaring the target’s emotions, making them favor or worship him

Recruitment: Semi-forced induction of subdued enemies into his ranks

Tempter is considered an advanced version of Rimuru’s former skill Heartless One, surpassing it in both scope and finesse.

This skill later evolves into, and is integrated within, his ultimate ability Lord of Temptation (Azazel).

Great Sage (Motomu-mono)

Great Sage is a thought-acceleration and analysis-type Unique Skill, strongly reminiscent of Rimuru’s Daikenja (Great Sage).

It essentially acts as Diablo’s internal super-intelligence.

Its abilities include:

Accelerated Thought: Processing and reaction speeds far beyond normal limits

Chant Annulment: Allows the casting of magic without verbal incantations

All-things Analysis: Comprehensive information gathering and interpretation of phenomena

Law Manipulation: Limited interference with and adjustment of underlying world laws

Like Tempter, Great Sage is eventually absorbed and unified into his Ultimate Skill, Lord of Temptation (Azazel).

After this, the original Unique Skill slots of Tempter and Great Sage disappear.

Lord of Temptation (Azazel)

Lord of Temptation is Diablo’s evolved Ultimate Skill, obtained after his awakening as a Devil Lord.

It combines and upgrades all functions of Tempter and Great Sage into a single, godlike authority.

Its capabilities include:

Accelerated Thought (greater than before)

Universal Perception: Sensing and understanding all phenomena in his surroundings

Demon Lord’s Haki: An aura that overwhelms and suppresses others, crushing their will to fight

Space-Time Manipulation → Space-Time Dominion: Advanced control over time and space, later upgraded to full dominion after battles such as the fight involving Michael

Multidimensional Barrier: Creating super-high-level barriers across multiple dimensions

All-things Analysis (Forest of All Things): Deep comprehension of reality, akin to omniscient analysis

Punishment Control: Variably administering “penalties” to targets under his authority

Charm Control: Master-level manipulation of charm and enthrallment

World of Temptation: Creation of a personal inner world that can engulf targets and subject them to his complete control

The combination of mental domination, reality manipulation, and space-time control makes Azazel one of the most broken abilities in the setting.

Very few beings possess the spiritual strength or conceptual defenses to resist it.

Techniques

Despair Time

Despair Time is a technique based on the World of Temptation aspect of his powers.

It drags a target into a fully realized pseudo-world where Diablo holds absolute authority.

In this constructed world, Diablo controls the target’s life and death, the flow of events, and the rules of reality.

Through “fiction-reality inversion”, what occurs inside the illusion can be swapped with the real world, making illusions become actual physical events.

This means any nightmare Diablo inflicts can be turned into material reality, a deeply unfair and terrifying capability.

The only way to escape is through sheer spiritual strength, overwhelming Diablo’s domination from within—but almost no one can match a Devil Lord–class demon king in spiritual fortitude.

Collapsing World (End of World)

Within the World of Temptation, Diablo can trigger Collapsing World, a technique that causes the pseudo-world to implode while trapping the target inside.

The collapse annihilates the target along with the world, acting as an inescapable execution.

Requiem to the End of the World (End of World Requiem)

Requiem to the End of the World is an ultimate hybrid technique combining ability, martial art, and magic.

It re-creates the spectacle of a world’s end to produce catastrophic localized destruction.

By harnessing elemental and illusion-based magic along with his world-manipulation powers, Diablo manifests a “miniature apocalypse” in a focused region.

This is considered his strongest and most dreadful finishing move in terms of pure destructive spectacle.

Quadruple Composite Extreme Art: Absolute Annihilation Nostalgia Break (Breakdown Nostalgia)

This technique is a four-person combination attack:

Diablo, Benimaru, Zegion, and Dino join forces to create a world-shaking offensive art.

Their combined power produces a destructive force greater than anything seen since the birth of the universe in their setting.

Used freely on a planet’s surface, it could obliterate not only the world but even engulf an entire solar system.

Celestial Circular Truth

Celestial Circular Truth is a high-level combat doctrine Diablo designs based on Rimuru’s slime cells and the concept of Void Collapse.

It optimizes the circulation and usage of energy to such a degree that resource consumption almost ceases to matter.

By reaching perfect efficiency and balance, this technique theoretically allows endless combat continuity.

With Celestial Circular Truth, Diablo—or Rimuru employing it—can fight any enemy indefinitely, making them practically invincible in protracted engagements.

Celestial End

Celestial End is a finishing move built on Celestial Circular Truth.

In this state, Diablo’s power is maximally stimulated through ultra-high-speed magic activation, and he then unleashes Requiem to the End of the World at its absolute apex.

This move is powerful enough to effortlessly erase an ultimate life-form whose existence value has reached several hundred million—such as Jahil, who had absorbed Vega and others like Kakeashi, Suim, and Zelanus.

It is one of the clearest demonstrations of Diablo’s destructive ceiling.

Magic and Weaponry

Diablo specializes in dark and elemental magic.

His control is so precise and overwhelming that many opponents never land a single meaningful blow.

He often manifests bladed claws known as Scissor Claws.

These appear to be an application of the demons’ racial skill Matter Creation, transforming his fingers into deadly cutting weapons rather than simply equipping an external tool.

In the main novels and anime, he is rarely seen using swords or other conventional weapons.

However, in the game The Legend of the Demon King and the Dragon’s Kingdom Building, he is depicted using swords and similar gear in battle, showing his versatility across media.

Within the Jura Tempest Federation, Diablo is one of Rimuru’s Three Peaks—the three strongest subordinates.

Even among these three, he is often considered the strongest in pure offensive potential and hax.

During his naming, half of Rimuru’s massively expanded energy—after his own awakening—was consumed, clearly marking Diablo as in a different league from other executives.

Later, after his awakening and acquisition of Azazel, his existence value rivals Rimuru’s former slime-spirit state.

He commands the Black Numbers, an elite legion of around 700 demons, including the Primordial trio Testarossa, Ultima, and Carrera.

In practice, he is the supreme commander of all demon forces within Rimuru’s nation, though the Primordials are treated more like peers than subordinates.

When serious, Diablo can eliminate awakened Demon Lord–class opponents in an instant.

His psychological warfare, illusion-reality inversion, and world-collapse techniques make him one of the most feared beings on the battlefield.

After the Tempest Opening Festival, Diablo requests “staff” from Rimuru—essentially subordinates he can dump chores on.

Rimuru obliges by providing about a thousand prepared vessels for demons, and Diablo returns to the demon world to recruit.

When he comes back, he brings approximately 700 demons, including three other Primordials: Testarossa (white), Ultima (purple), and Carrera (yellow).

All three are captivated by Rimuru and pledge allegiance, much to Diablo’s satisfaction.

Rimuru names this elite corps the Black Numbers, making them Tempest’s greatest strategic force.

Though Diablo is technically their overall leader, his relationship with the Primordial trio is somewhat lateral, with plenty of friction and comedic bickering.

Diablo has to intervene when Ultima and Carrera start terrifying “gang wars” with each other.

Although he is annoyed that this wastes time he’d rather spend with Rimuru, he ultimately drags them into the labyrinth and beats them down to restore order.

His favorite subordinate among the non-Primordials is Venom, an ex-human otherworlder turned demon.

Venom’s repeated challenges despite the vast power difference amuse Diablo, and he sees potential in him.

Encounter with Shizue Izawa

Decades before the main story, when Shizue Izawa was still an active hero, Diablo is summoned by an unnamed adventurer in exchange for that person’s soul.

The request is an assassination job in the kingdom of Filwood.

Using the temporary alias “Black”, Diablo infiltrates Filwood.

There he encounters Shizue and ends up fighting her.

Shizue, using the power of Ifrit (Flame Giant), manages to actually hurt Diablo, which intrigues him.

But when he touches the mysterious anti-spirit mask she wears—a mask created from future-origin materials—it violently reacts and blows off his arm.

Instead of infuriating him, this experience greatly piques his curiosity, especially about the mask and its origin.

He decides to spare Shizue, withdrawing from the fight.

Later, Diablo confronts the true mastermind, Ortos, an arrogant red-affiliated magic user who has sided with demons but does not realize Diablo’s true nature.

After Ortos insultingly calls Diablo “low-class,” Diablo crushes not only his body but also his soul.

Diablo then slaughters the enthralled king, ministers, and many soldiers who were puppets under Ortos’s scheme.

He and Shizue coordinate their story afterward, falsifying the events so the truth is concealed from others.

In return for Shizue’s cooperation in the cover-up, Diablo promises not to target her again.

This incident becomes one of Shizue’s lingering regrets, and in hindsight it also binds Diablo indirectly to Rimuru’s fate.

Spin-offs like The Slime Diaries suggest that Diablo and Shizue crossed paths multiple times afterward as well.

Shizue’s student Alice even sees through his true nature early on, recognizing that he is not “just a normal demon.”

Observation of Rimuru

When the anti-spirit mask finally breaks and Shizue’s life reaches its end, Diablo realizes that the “future-made” artifact has run its course.

He sees Rimuru inherit Shizue’s form and will, and he becomes convinced that Rimuru is an extraordinary being.

From that point on, Diablo observes Rimuru from the shadows, watching the nascent monster nation grow.

In the Slime Diaries depiction, by the time the first major arc ends, Diablo can already talk about Rimuru’s charm at “two-page spread” length internally.

He recognizes Rimuru as “that person” he has been waiting for, someone truly worthy of his absolute service.

Thus, when the opportunity arises during the Falmuth war, he forces his way into Rimuru’s summoning.

Falmuth Kingdom War – “Holy-Demon Confrontation Arc”

During the Falmuth Kingdom’s invasion of Tempest, Rimuru performs a large-scale demon summoning as part of his Demon Lord ascension plan.

Diablo answers this call in place of lower-ranked demons, disguising himself as a High Demon to bypass the summoning’s limitations.

After Rimuru awakens as a Demon Lord and returns, he tasks Diablo with handling the remaining enemies and political maneuvering.

Diablo infiltrates Falmuth’s aristocracy, manipulating nobles behind the scenes, but the plan is disrupted when Reyheim, who had been turned into a puppet by the Seven Days Clergy, is killed for silencing.

Diablo takes this as an unexpected failure and is genuinely depressed about the blunder.

He is then ordered to confront the enemy headquarters and, per Rimuru’s instructions, reluctantly attempts peaceful negotiations first.

However, when the new Falmuth king Edward Falmuth—ignorant of the full truth—insults and slanders Rimuru, Diablo loses his patience.

He unleashes his Demon Lord’s Haki, casually drops references to other Primordials like “Guy” and “Blanc,” and then reveals to the informed ones that he has evolved into a Demon Peer.

The Demon Hunter elites, along with warriors like Saare and Glenda Attley, quickly realize they are outclassed and lose the will to fight.

The real culprits, members of the Seven Days Clergy, show up to silence witnesses and end up facing Diablo.

He easily swats aside their attacks when they attempt to kill him without knowing who he truly is.

Then, once they reveal enough to expose themselves, he kills them instantly in front of journalists and soldiers, laying bare the conspiracy behind the war.

Diablo then offers a deal: he will protect the Falmuth soldiers and journalists, but Edward himself is excluded.

He tells Edward that if he abdicates and hands the crown to Youm Farmenas, he will be allowed to live—as a powerless former king.

This effectively forces a total surrender of Falmuth and installs Youm as the new monarch under Tempest’s influence.

Diablo’s actions thus reshape the political landscape of the Western Nations in a single move.

Tempest Opening Festival – “Magical Capital Open-Country Arc”

During Tempest’s grand Opening Festival, Diablo is appointed one of the Four Heavenly Kings, a ceremonial set of top representatives.

He trembles with joy at this honor, treating it as proof that Rimuru trusts and values him.

He also serves as a referee in the martial tournament, carefully observing matches like Geld vs. Karion and the bouts involving Honjou Masayuki and Gobta.

He takes these duties very seriously, seeing them as another way to serve Rimuru and safeguard the festival’s security.

Creation of the Black Numbers – “Demon Schemes Arc” / “Hero Awakening Arc”

After the festival, Rimuru wants to reward Diablo for the successful Falmuth operation.

Diablo asks for subordinates to whom he can delegate routine tasks, so he can himself focus more closely on important duties near Rimuru.

Rimuru prepares about 1,000 vessels suitable for demonic possession and sends Diablo to the demon world.

Diablo then recruits (or “kidnaps,” depending on your perspective) demons for Tempest.

When he returns, he presents Rimuru with around 700 demons, including the Primordials Testarossa, Ultima, and Carrera.

All three instantly pledge fealty to Rimuru upon seeing him, much to Diablo’s satisfaction.

Rimuru names this army the Black Numbers, establishing them as his nation’s ultimate trump card.

The story later confirms that, in external materials, the demons who might have been summoned instead of Diablo were “eradicated” by him beforehand, with Diablo casually remarking that he left them “unable to complain.”

Battle vs. Rain – “Hero Awakening Arc”

During a music exchange event in Luberios, Diablo accompanies Rimuru.

Inside the great cathedral, he faces a bug-type demon named Razul.

Mid-fight, Diablo senses someone teleporting into the outskirts of the city and is ordered by Rimuru to investigate.

He finds Rain, the Primordial Blue and retainer of Guy Crimson, waiting in the open field.

Rain is using her “Avatar” ability, meaning the Diablo she’s facing is not her true body but a remote projection.

Not wanting to waste time away from Rimuru, Diablo immediately uses an ultimate holy magic, a multi-stage “Spirit Particle Collapse,” and ends the fight swiftly.

Afterward, he runs into Guy Crimson himself and the two rekindle their old dynamic.

Diablo enthusiastically explains that he has recruited Testarossa and the others into Rimuru’s faction and starts waxing poetic about the happiness of serving Rimuru, causing Guy to be deeply annoyed and mentally exhausted.

War Preparations – “Night Before War Arc”

In Tempest, a high-level strategy meeting is held with major powers like Leon Cromwell and Luminous Valentine, focusing on the coming war with the Eastern Empire.

Guy arrives uninvited, demanding an explanation for Diablo bringing other Primordials into Rimuru’s camp and possibly upsetting the world’s power balance.

Guy cheerfully appears with a casual “Yo!” in the conference room.

Diablo responds by curtly telling him to “Get out,” and physically ejects him from the room—only later allowing his return after Rimuru mediates.

During the discussion, Rimuru hesitates to explain his ideals to Guy, judging that preaching “idealism” to the world’s balancer is a bad idea.

Diablo steps in and, with relish, describes Rimuru’s vision.

He explains Rimuru’s desired world: an economic and security-based network of mutual prosperity, where others become dependent on Tempest’s infrastructure and protection.

As the world becomes reliant on Tempest, the resulting gratitude and deference will allow Rimuru to indirectly control humanity, which Diablo contrasts with Guy’s more direct “rule through fear” model.

Diablo argues that this gratitude-based, soft-power domination is more efficient and stable than raw terror.

He does so with almost religious ecstasy, painting Rimuru’s future empire as a beautiful inevitability.

After Guy leaves, Diablo receives a report from Razen in Falmuth: the legendary mage Gadra wishes to defect and swear loyalty to Rimuru.

Diablo takes a moment to consider the implications, then informs Rimuru, leading to Gadra becoming a vital inside source on the Empire.

Eastern Empire Invasion – “Empire Invasion Arc”

When war breaks out with the Eastern Empire, Diablo is initially tasked with guarding Rimuru in the Dungeon (Labyrinth).

However, as he senses his comrades fighting and gaining glory, he longs to join the fray.

After receiving permission from Rimuru, he teleports to the battlefield.

He quickly encounters Jiwu and Barney and eliminates them with ease, practically in passing.

A girl named Misha, associated with Yuuki Kagurazaka, is spared for strategic reasons rather than personal mercy.

Diablo then heads straight for the Empire’s command center.

There, he confronts the high-ranking officer Caligulio and his army of more than ten thousand troops.

In a single moment, Diablo harvests the souls of over ten thousand soldiers, collapsing the army and driving Caligulio to despair.

Enraged and on the edge of madness, Caligulio awakens as a Saint and reaches awakened Demon Lord–level power.

Even so, Diablo demolishes him effortlessly—his awakening changes nothing in the face of Diablo’s overwhelming might.

Twelve Guardian Lords and Awakening – “Dragon-Demon Clash Arc”

During a later Theoretical Conference (Rongkoushiki), Rimuru formally appoints the Twelve Guardian Lords, top guardians of his nation.

Diablo is one of them and receives the title Demon Lord (Magin King).

As part of Rimuru’s strategic preparations, Diablo is granted the power of 100,000 souls.

This triggers his evolution from a Demon Peer into a full Devil Lord (Devil Road), and his skills ascend into the Lord of Temptation (Azazel) class.

After this evolution, his magicule level becomes comparable to Rimuru’s old slime-spirit phase.

Some characters speculate that, if Rimuru holds back or in certain situations, Diablo might actually be stronger in direct confrontation.

In one meeting, when the “game” between Guy Crimson and Emperor Rudra is mentioned, Diablo casually summons Guy to ask for the rules and victory conditions.

Later, when Guy decides to stay overnight in Tempest, Diablo seizes the chance to drag him to a private room and bombard him with questions and bragging about his own ultimate skill, to the point that Guy is thoroughly fed up.

Trap of the Dream Fortress

When Rimuru goes to the Empire to negotiate peace, Diablo and other executives accompany him.

However, the group is diverted by a trap and teleports into Veldora Tempest’s sister Velgrind’s construct, the Dream Fortress.

They find themselves trapped in an alternative dimension meant to separate and delay them.

Diablo participates in the subsequent clashes within that realm and continues to refine his control over space-time and his ultimate skill through these battles.

Diablo appears across multiple media related to That Time I Got Reincarnated as a Slime, including:

The main light novels

Manga adaptations

Anime series and OVAs

The spin-off series The Slime Diaries

Games like The Legend of the Demon King and the Dragon’s Kingdom Building

In 2024, he also appears in the Granblue Fantasy collaboration event “That Time I Got Reincarnated as a Slime: Azure Bond Arc.”

He is implemented as a limited-time playable gacha character with dark attribute, the same as Rimuru and Shion in that event.

This allows players to form a “Slime” themed team featuring Rimuru, Diablo, and Shion, with Veldora Tempest as a wind attribute unit.

His depiction in this and other spin-offs leans into his overpowered nature and his obsessive butler-like devotion to Rimuru, which have become his most iconic traits.