Diablo canon, not “Legends” in the Star Wars sense
Current Warhammer 40K continuity
Lore-friendly to both Diablo and 40K
Lore-purist standalone analysis
Tier-list usable

Diablo does not really have a clean Canon vs Legends Sidious split. The closest useful split is:

Standard canon form — what the character normally is in the mainline Diablo lore.
Peak/composite canon form — for example, Diablo as the Prime Evil after absorbing the other Great Evils in Diablo III.
Blizzard’s current Diablo IV framing has Lilith defeated, Mephisto active as the Prime Evil of Hatred, and Hell’s forces continuing to threaten Sanctuary, so the safest crossover assumption is mainline Diablo canon, with “Prime Evil Diablo” treated as a separate high-end form. Blizzard’s official Lord of Hatred page frames Sanctuary as a refuge born from rebellion between Heaven and Hell, confirms Lilith’s defeat, and identifies Mephisto as the Prime Evil of Hatred. (Diablo IV)

The Burning Hells in Warhammer 40,000

Lore-Purist Survival Analysis

Continuity assumptions

This analysis uses current 40K continuity, meaning Cadia has fallen, the Great Rift/Cicatrix Maledictum has split the galaxy, Guilliman’s Indomitus Crusade is active, Necrons are advancing with blackstone pylons, and the Imperium is fighting under extreme strategic strain. Warhammer Community summarizes this era as one shaped by Cadia’s destruction, the Cicatrix Maledictum, the Indomitus Crusade, increased Necron activity, and blackstone pylon networks that can cut regions off from the Warp. (Warhammer Community)

The important rule is this:

Diablo demons are not automatically Chaos daemons.

They are “demon-adjacent” from the Imperium’s point of view, but their native metaphysics are different. Diablo’s Burning Hells are part of a Heaven/Hell/Sanctuary cosmology, while 40K daemons are Warp entities shaped by emotion, belief, and the Chaos Gods.

So the most lore-friendly interpretation is:

The Imperium misclassifies them as daemons.
Chaos sees them as rivals, prey, tools, or contamination.
Grey Knights and Ordo Malleus weapons can hurt them, but may not banish them cleanly like native Warp daemons.
Necron blackstone, nulls, Sisters of Silence, and anti-psyker measures are still serious threats if Diablo magic is treated as immaterial/soul-based power.
That is the strongest crossover framework.

How the Imperium would classify them

The Imperium would probably classify Burning Hells entities as one of the following:

The Inquisition would get involved quickly. The Ordo Malleus exists to safeguard the Imperium from daemons and Warp influence, while the Ordo Hereticus hunts rogue psykers, mutants, heretics, and internal corruption. (Warhammer Community)

The Grey Knights are the obvious military response if the threat is treated as daemonic. Warhammer Community describes Ordo Malleus daemon-hunters facing Greater Daemons, with Grey Knight Terminators strong enough to send them back to the Warp. (Warhammer Community)

Core crossover principle

The Burning Hells survive in 40K if they do three things:

Avoid being absorbed into the Chaos ecosystem.
Avoid becoming visible enough to trigger a full Ordo Malleus/Grey Knights response.
Build cults, fear networks, soul-economies, or hidden domains before the Imperium understands what they are.
They lose if they try to fight 40K like a normal Diablo campaign.

40K is too big, too militarized, too paranoid, and too accustomed to daemonic war.

Lilith in 40K

Standard canon Lilith

Lilith is not a simple destroyer. She is a revolutionary, manipulator, mother-figure, cult-founder, and civilization-shaper. In Diablo lore, she is tied to Sanctuary’s creation and to the idea of humanity being more than a pawn of Heaven or Hell. Blizzard’s current material continues to frame her legacy as central to Sanctuary’s struggle after her defeat. (Diablo IV)

That gives her a very strong 40K survival profile.

She would not begin by attacking Terra or challenging Guilliman. She would begin among the abandoned:

underhive populations,
mutant communities,
refugee columns,
isolated shrine worlds,
desperate Guardsmen,
war-torn Imperial worlds,
worlds cut off by the Great Rift.
Her message would be dangerous because it would sound almost reasonable:

“The Imperium consumes you. Chaos enslaves you. I offer a third path.”

That is exactly the kind of ideology the Ordo Hereticus would burn out immediately if detected.

Strengths in 40K

Lilith’s best advantages are:

cult formation,
emotional manipulation,
blood magic,
symbolic motherhood,
anti-authoritarian appeal,
ability to exploit Imperial cruelty,
hatred of being controlled by higher powers.
She could become a major heresiarch figure without technically serving Chaos. That makes her especially dangerous because Imperial authorities would struggle to decide whether she is a Chaos cult leader, xenos contaminant, rogue psyker, or independent daemon-class entity.

Weaknesses in 40K

Her biggest problems are:

Grey Knights,
Sisters of Silence,
Culexus Assassins,
blackstone/null fields,
Inquisitorial surveillance,
Chaos trying to claim or destroy her,
her own pride,
and the fact that “protecting humanity” in 40K often requires tolerating horrors she would despise.
The Sisters of Silence are especially bad for her if her power is treated as psychic/soul-based. Warhammer Community describes the Pariah gene as making the Sisters of Silence ideal counters to warpspawn and deadly psykers. (Warhammer Community)

Lilith verdict

Lilith could survive for a long time if she stays subtle. She could build a hidden anti-Imperial, anti-Chaos movement around the abandoned and oppressed. She is not likely to conquer the galaxy, but she could become a sector-level ideological and supernatural crisis.

Tier placement: A Tier, possibly low S if she successfully builds a durable cult network.

Diablo in 40K

Standard canon Diablo

Diablo is the Lord of Terror, and that matters enormously in 40K. Fear is everywhere. Hive cities, voidships, battlefields, penal worlds, shrine worlds, daemon worlds, plague zones, and frontier colonies are all saturated with terror.

Blizzard’s Diablo II recap identifies Diablo as the Prime Evil whose power can survive defeat through possession, with the hero of the first game becoming the Dark Wanderer after embedding Diablo’s soulstone into himself. (Blizzard News)

That gives Diablo a very strong survival mechanic: he does not need to win every fight physically. He needs vessels, fear, cults, and time.

Strengths in 40K

Diablo’s greatest asset is not raw firepower.

It is that 40K is psychologically perfect feeding ground.

He could feed on:

fear of Chaos,
fear of mutation,
fear of xenos,
fear of the Inquisition,
fear of the Emperor’s judgment,
fear of starvation,
fear of damnation,
fear of abandonment in Imperium Nihilus.
A hive world alone could sustain him for centuries if he remains hidden.

Weaknesses in 40K

His problem is that fear in 40K is already spiritually contested territory. Chaos feeds on emotion. The Warp echoes fear, rage, despair, ambition, obsession, and suffering. Diablo entering that ecosystem would not give him a free buffet; it would put him in competition with the gods and their servants.

Tzeentch would study him. Khorne would test him. Slaanesh would tempt him. Nurgle would exploit despair around him. The Black Legion or Word Bearers would try to bind, bargain with, or weaponize him.

The Imperium would eventually treat him as a daemon-class existential threat.

Standard Diablo verdict

Standard Diablo could become a long-term 40K crisis if he operates through possession, fear-cults, and hidden terror networks. He is far more survivable than most horror villains because he can become a recurring metaphysical infection rather than a single battlefield monster.

Tier placement: S Tier.

Prime Evil Diablo

This is the high-end form that should be separated from standard Diablo.

Prime Evil Diablo, the version associated with the combined essence of the Great Evils in Diablo III, is a much larger threat. This is the closest Diablo gets to a “Legends Sidious” style escalation.

In 40K terms, Prime Evil Diablo is not just a greater daemon-equivalent. He is closer to a major daemon-primarch / greater-daemon-plus / sector-ending metaphysical threat.

But he still does not automatically beat 40K.

Why?

Because 40K has:

Chaos Gods,
daemon primarchs,
C’tan shards,
Necron anti-warp technology,
blackstone pylons,
Grey Knights,
Sisters of Silence,
exterminatus protocols,
mass attrition warfare,
and factions that already fight gods, daemons, and reality-breaking entities.
Prime Evil Diablo could devastate worlds and become a galactic crisis. He could not simply walk through the entire setting unopposed.

Prime Evil Diablo verdict

Prime Evil Diablo is S Tier or S+ for your list. He is one of the few horror/fantasy entities who can plausibly become a galaxy-level crisis in 40K, but even he eventually collides with Chaos, Necron countermeasures, and Imperial daemon-hunting systems.

Tier placement: S Tier, above standard Diablo.

Mephisto in 40K

Mephisto may be even more dangerous than Diablo strategically.

Hatred is everywhere in 40K. The Imperium runs on hatred of the alien, the mutant, the heretic, and the witch. Chaos warbands hate each other. Xenos empires hate or fear humanity. Civil wars and purges are constant.

A Lord of Hatred would not need to invent anything.

He would amplify what is already there.

Mephisto would likely avoid open war and instead work through:

hate-preachers,
shrine cults,
pogroms,
anti-mutant movements,
radical inquisitors,
revenge-driven commanders,
fractured Space Marine successor politics,
anti-xenos crusade hysteria.
Blizzard’s current official Diablo IV expansion framing puts Mephisto’s power at the center of the next crisis, spreading corruption while his influence threatens Sanctuary. (Diablo IV)

Mephisto verdict

Mephisto is probably the most 40K-compatible of the Prime Evils. Diablo feeds on fear, but Mephisto can weaponize hatred, and 40K is almost entirely built from organized hatred.

Tier placement: S Tier. Possibly above standard Diablo for long-term infiltration.

Baal in 40K

Baal, Lord of Destruction, is powerful but less subtle.

He is dangerous in war zones, crusades, and collapsing planets. He would thrive around:

orbital bombardments,
siege worlds,
daemon worlds,
exterminatus campaigns,
Tyranid invasions,
Ork wars,
Imperial civil wars,
Black Crusade battlefronts.
But destruction is also the easiest pattern for the Imperium to notice. A Baal outbreak looks like an apocalyptic war-front, and 40K has many tools for dealing with apocalyptic war-fronts.

Baal verdict

Baal is immensely destructive but less survivable than Mephisto or Diablo because his domain encourages escalation. He can ruin worlds, but subtle long-term survival is harder.

Tier placement: A Tier or low S Tier.

The Lesser Evils in 40K

Belial, Lord of Lies

Belial translates extremely well into 40K. Lies, false visions, forged orders, corrupted records, false saints, fake inquisitorial mandates, and manipulated astropathic messages are all perfect tools.

He could do massive damage without ever appearing physically.

Tier placement: S or high A.

Azmodan, Lord of Sin

Azmodan is a warlord and strategist. He could build armies, corrupt military hierarchies, and lead campaigns. But 40K is filled with better-established war gods, daemon primarchs, Chaos lords, and immortal generals.

He is dangerous, but he has competition.

Tier placement: A.

Andariel, Maiden of Anguish

Andariel works best as a terror/regional corruption entity. She could dominate torture cults, slave-pits, underhives, Drukhari-adjacent nightmares, or Chaos-tainted pain cults.

But she is less strategically flexible than the Prime Evils.

Tier placement: B to A.

Duriel, Lord of Pain

Duriel is brutally dangerous in direct combat and localized horror scenarios, but he is not a galaxy-scale manipulator. In 40K, direct brutality alone is not enough.

Tier placement: B.

Burning Hells armies in 40K

A full Burning Hells incursion is more dangerous than any single demon.

The Hells bring:

endless demonic infantry,
soul corruption,
possession,
hellfire,
war beasts,
cults,
siege creatures,
infernal magic,
realm-based recurrence.
But they face a setting already optimized for daemonic invasion. The Imperium has fought Chaos for ten thousand years. Grey Knights, Ordo Malleus, Sisters of Battle, Inquisitors, Ecclesiarchy cults, sanctioned psykers, nulls, and exterminatus protocols all exist because 40K expects supernatural war.

So a Burning Hells invasion could absolutely destroy worlds, but conquering the setting is unlikely unless Hell establishes a stable metaphysical foothold.

Burning Hells army verdict

A Burning Hells incursion could become a sector-level catastrophe. A sustained portal network could become a major galactic problem. But without adapting to the Warp, Chaos, null-tech, blackstone, and Imperial containment doctrine, Hell’s armies eventually get treated as another daemon warfront.

Tier placement: A as an invasion force; S if they establish a permanent realm-bridge.

Corrected tier-list additions

For your existing Horror Villains in 40K Survival Tier List, I would add them like this:

S Tier — Can become a long-term 40K crisis

Prime Evil Diablo
Mephisto
Standard Diablo
Belial
Burning Hells invasion with stable portal network
A Tier — Can destabilize worlds or sectors if subtle

Lilith
Baal
Azmodan
Burning Hells warhost without stable portal network
B Tier — Lethal local threats, weak strategic scaling

Andariel
Duriel
Lesser Hell demons / elite demon packs
C Tier — Needs a niche, patron, cult, or technology to matter

Individual low-tier demons cut off from Hell
Possessed cult leaders without major infernal backing
I would not put Lilith automatically in S Tier unless you are allowing her to build a durable cult civilization. She is extremely dangerous, but her refusal to submit to anyone makes her a target for both the Imperium and Chaos. That independence is morally interesting, but strategically risky.

verdict

The Burning Hells would survive in Warhammer 40K, but not by replacing Chaos or overpowering the galaxy outright. Their strongest figures would adapt by becoming hidden infections: fear cults, hatred movements, false martyrdoms, infernal bargains, soul-contagions, and underhive religions. Diablo, Mephisto, Belial, and Prime Evil Diablo have S-tier survival potential because their domains map perfectly onto 40K’s emotional ecosystem. Lilith is slightly lower because her independence and protective self-image make her harder for Chaos to absorb, but also harder for her to compromise. She could become a powerful anti-Chaos, anti-Imperial heresiarch — not a savior, but a third-path threat.