The Prince of Emptiness is a relatively new demon lord of the Abyss. While its exact age is unknown, whispers of its existance only staring reaching mortal ears within the last century or two. Demon lords are born and destroyed all the time in the Abyss but few can withstand the test of time; thus, the longevity of Ar-Manus is of note to demons and demonologists alike.

The revelation of Ar-Manus as a golem is puzzling. The vast mystery of how a golem managed to achieve what untold numbers of demons have tried and failed to do is mind-boggling as is its origins, which are shrouded in mystery. Recent expeditions have also revealed the alarming presence of mind flayers hidden within its Layers of the Abyss. Though the level of their co-operation is unknown, many see - and rightfully so- the idea of the ithilids and demons working in tandem as a crisis.

Ar-Manus is massive in size even among demon lords. Standing a monstrous 28 feet tall sporting a thick, black, armored shell of iron golem, the helm that makes up its head is crowned by two pairs of horns stolen from slain ancient, red dragons and possesses a screaming, demonic visage along with four, piercing red lights for eyes that seem to hold an unfathomable hatred it could not possibly feel. It wields two, massive greatswords as its strength is so mighty and control so flawless it can dual wield these immense weapons. Upon its hunched back reside a dark cannon fused with its hulking body.

None are certain how much of a mind there is behind the ever-advancing madness, but its apparent alliance of mind flayers and reports of its psychic abilites paint a terrible picture.

FULL DESCRIPTION

If you desire the long backstory and worldbuilding designed for this demon lord. It is based on information from the Out of the Abyss module and Mordenkainen's Tome of Foes.

Origin

Ar-Manus was once a nameless, iron golem that served an adventuring archmage untold centuries ago. During an expedition into the 664 Layer of the Abyss, the party had been ambushed by demons and bade their escape with the iron golem being left behind to buy them time. Somehow, despite the onslaught the golem survived and continued fulfilling its master’s final orders: slay every demon you see. And so, it fought without end upon that layer demon after demon, ironically acting without a mind in the same manner a demon would in regard to its reckless slaughter.

Eventually, the chaotic essence of the Abyss corrupted it. Contorting its programming and its power into something that was both golem and demon: a soulless construct bound in the guise of a soul-consuming demon. At some point in this process – be it by a twist of fate or a freak backlash of a spell – a pilgrimage of ithilids led by an Ultharid, the foundation of a new colony, found themselves in the golems midst.

Despite its corruption, it had no interest in killing them as its last orders had been to kill demons. The ulitharid, sensing the opportunity, decided to lead its fledging colony on the golem’s warpath and thus did the ulitharid and the golem became bound.

Eventually, with the golem gaining the benefit of illithid technology and powers, it became chosen by the Abyss as the demon lord of the 664 Layer. But what could the mindless being mold a realm into? What could a being void of imagination craft in chaos? The ulitharid had the answer, for it and its kin, too, had started to become corrupted and molded by the Abyss they inhabited, and now bound to the demon lord of the layer molded it into one suitable for the demonic illithids. But what had the Abyss’s corruption done to affect this? Were the mind flayers twisted by the demonic essence or enhanced by it? Were they in control of the demon lord or was it in control of them? Or perhaps the relationship was sympathetic? None truly know.

664 Layer of the Abyss

Otherwise known as the Hadean Circle, it is a floating continent propelled over a vast whirlpool of darkness and magma, the island itself being a perfectly-circle with an adamantium ring as its border. A never-ending lightning storm encircles the skies in a fierce maelstrom. Active volcanoes dot each quadrant perfectly mirroring the others while metallic canals in the dark, stone ground allow lava to flow directly to the center of the continent. At its center is a ring of eight super volcanoes each orbited by eight smaller ones. Interconnecting this mountain range of gushing flame is a massive, ringed complex known as the Dark Factorium.

The Dark Factorium is a circular complex of such scale that overshadows the eight super volcanoes and sixty-four smaller volcanoes supporting its vast structure. Obsidian pillars of adamantium piercing down the centers of the fiery peaks which are overcome by tiered structures that surround the pillars. Layers of metallic causeways and other attached structures litter the pillars in an organized fashion up unto the zenith of the pillars which possess a long spike to which lightning is ceaselessly attracted to. Flying buttresses extend of the main structure at the volcano’s peak to form smaller, offspring pillars for the surrounding smaller volcanoes with the design replicated below on a smaller scale.

Each of these eight primary structures has a massive causeway that leads to an obsidian, tiered fortress set upon a massive pillar. These fortresses around interconnect by set of additional causeways that are interspersed with fortified towers at each change of angle. These towers possess a platformed ring around their crown which leads to another set of causeways leading to a traversable ring. The outer edge of this ring is encompassed by merlons made up of individual, turreted towers. This ring is in turn connected by four, large causeways to a massive, obsidian structure at its center. Three of these causeways lead to ziggurats that hug the edge of the tower. The northern causeway leads into a castle that makes the gatehouse to the lynchpin of the entire complex: an obsidian-colored, adamantium tower that reaches from a gargantuan stronghold that crowns the entire realm gazing down at the volcanic peaks below to the swirling whirlpool of lava that makes up the true center of the Abyssal Layer and where all lava-flow canals drain into.

This complex’s design, despite the demonic implication and architectural planning that might’ve brought envy to dwarvish minds, is littered with qualith and other indications of illithid presence.

Ar-Manus Elder Brain Symbiosis

The ulitharid who bound it and its followers to the Ar-Manus centuries ago – its name forever unknown – has since, with its brethren, formed an elder brain deep within the confines of the Dark Factorium. Whether by the chaotic essence of the Abyss or through its connection to the demon lord itself or both, it has become corrupted into a demonic visage of an elder brain. The symbiosis between the golem and the brain has led to an exchange of characteristics that together form a union of bizarre logical chaos. The elder brain lost its ego and arrogance in favor of an emotionless rational that aids its golem counterpart in disregarding its corrupted programming. Conversely, the golem gained in itself a drive to achieve and the ability to defy its own programming. In this will has begun to develop an intelligence learned from its counterpart.

The ithilaurs, the demonic mind flayers, that serve the elder brain, are also affected by the voidness within the golem and have subsequently become agents acting in the demon lord’s stead beyond the Dark Factorium. Though to call these three separate beings: the golem, the elder brain, and the ithilaurs ‘separate’ in the traditional sense is a bit misleading as the chaotic energies residing in the Abyss have made that link between them more tangible than even normal illithids could even imagine.

Due to this, the current objective of Ar-Manus has been noticeably clear. It has been gathering a vast amount of resources from its own and various other planes; its forces enacting violent raids against resource shipments, mines, and other such places to steal metals and other material. Unlike most of its demonic kin, invasions seem to be nonexistent for the Prince of Emptiness. Keepers of the Balance have come to a dreadful realization because of demon lord’s strange behavior. Initially, it was believed that the immense increase in the demon lord’s size from what it was as an iron golem was due to the Abyss’s malleable chaotic energies and its ‘ascension’ into becoming a demon lord: that it is gathering the resources to increases its size. If this theory holds true, there could only be one reason why. It is known that the elder brain and the golem are mentally bound. Therefore, it stands to reason they would seek to make this bound a physical one as well. Should this come to pass, and the elder brain and the golem unify, it could tip the balance of power dangerously towards a precipice from which it will never recover.

The Blood War

The war between the demon lords of the Abyss and the rulers of the Nine Hells is largely not a concern to Ar-Manus, and the demon lord hardly seems interested in pursuing the conquest there or against other demon lords. While it has ordered raiding in other layers, it tends to stay within the Dark Factorium guarding the elder brain it is connected with. The demon-corrupted mind flayers that service the elder brain act as agents of it, commanding the demons that serve it in its stead.

They have managed to steal knowledge concerning devilish constructs and they have even ripped knowledge on the creation of drow retrievers, spider constructs operated by the distilled spirit of a demon, from Lolth’s minions. They have used this knowledge to transform many of the demons under Ar-Manus’s sway into construct-infused abominations. While many demons that serve Ar-Manus are purely organic though they are typically are heavily armored in some capacity and use advanced weapons in combat.

This has led to most of Ar-Manus’s enemies being other demon lords, primarily Lolth and Baphomet. Lolth despises Ar-Manus for stealing the knowledge of the retrievers and considers it an immense insult. Her drow are often rewarded for major slights against the illithid-bound demon lord. The reason for the Prince of Savagery’s hatred of Ar-Manus is not based on a slight but more so because Ar-Manus is his antithesis. The hulking behemoth of a golem, despite being a chaotic demon lord, is likely the most civilized and non-instinctual of all the demon lords which is direct opposition to the primal nature of Baphomet.

Despite the opposition, there are  some demon lords that hold minimal or even no aggression towards the fledgling demon lord. Yeenoghu in particular seemingly enjoys its unrelenting demon hunt even in its current state and due to being in direct opposition to Baphomet has formed an unlikely alliance with the golem demon lord. It’s has even been noted that there have been instances of gnolls supporting the operations of Ar-Manus’s cultists.

The Nine Hells see this new demon lord as an opportunity to get one over on their eternal adversaries often coaxing the Spider Queen and the Prince of Savagery against Ar-Manus or providing Ar-Manus with the resources its desperate to collect in order to keep it from seeking resources in places that would be detrimental to them. They seem to be unaware of, or at least not taking seriously, the threat the new major demon lord may have on them.

For those that observe the Blood War and seek to maintain the stalemate, the rising of Ar-Manus poses a unique challenge that they desperately seek to resolve. Even without the unification of the golem and elder brain, it is still immensely powerful. Certainly, amongst the hardest demon lords to actually kill. With the size it’d have to be to comfortably accommodate the elder brain and the idea that the demon lord, in that state, would be able to freely leave its layer no longer bound by its self-restriction have raised terrible implication for the future.

Therefore, an effort has been made to stall the gaining of resources to slow this ‘ascension’ as it were. No attempt has been made to raid the Dark Factorium itself due to the psychic nature of their adversary as well as the fact that the demon lord is ever present in the facility. Most of their attempts come down to dealing with demon cultists that worship the Prince of Emptiness as a means of slowly the demon lord’s supply.

Cultists to Ar-Manus

Due to how young of a demon lord Ar-Manus is, it does not possess as many cultists as say Orcus, Demogorgon, or Baphomet. However, due to the nature of its existence and that the illithids bound to it act as extensions of its unified will, they are able to bring people under the thrall of Ar-Manus comparatively more easily that even the more ancient demon lords can as they can easily delve into the minds of the hapless fools who either call upon them or unfortunate enough to cross paths with them.

Those that end up worshipping the Prince of Emptiness quickly lose all empathy and their personality is dulled immensely, their ambition drowned out by the voices of Ar-Manus calling them to fulfill a higher purpose. Those under Ar-Manus’s sway tend to seek out plate armor or will replace limbs with metal armaments. Some become so zealous in this practice that they may mutilate their entire body to a horrify degree to more physically match their lord. Prolonged efforts in this might even result in their body being reshaped into something akin to an armored flesh golem or other atrocious abomination.

The goal of these cultists is often to gather metals such as iron or other material for transport to the Abyss. Entire mining towns or the nobles who own them are often major targets of cultists or for the influence of the demonic mind flayers who help corrupt them. Merchants and mercenaries may also become throngs to the twisted minds of the demon lord. The former due to their ability to purchase required materials or mercenaries to raid for such materials. Often, they call upon the demon lord with the promise of an unrelenting will, wealth, or other such wish but the dominating power of demonic essence combined with the psychic expertise of the mind flayers is not something to be underestimated. Only those with the greatest mental fortitude may gain such desires from Ar-Manus and even then, they are always at risk of succumbing.

Those that do seek a demonic boon from Ar-Manus and have the mental fortitude to resist becoming just another cog in its abyssal machine may yet grant it as a reward for overcoming its ‘test’, though the truth of the matter is that this 'gift' is simply to make it easier to break their mental resilience.

The following entries outline boons that a DM can grant to monsters and NPCs dedicated to a particular demon lord. The entries also list signature spells associated with a demon lord. If the monster or NPC can cast spells, you can replace any of those spells with spells from that list, as long as the new spell is of the same level as the spell it replaces.

A typical demon can impart boons to a number of creatures equal to the demon’s number of Hit Dice. In contrast, demon lords have no limit on the number of creatures that can receive their boons.

Boons from demons are fickle gifts. They remain in place only as long as the demon is pleased. Accepting such a boon is a damning act that corrupts the soul and drives a person toward acts of chaos, evil, and madness. Rejecting a boon likely provokes a demon’s wrath.

Other Mind Flayer Colonies

Unlike normal mind flayers, the demon mind flayers bound to Ar-Manus do not break free of the elder brain's control when out of its range of its direct link. While there is certainly a vast range they must travel before they no longer are interconnected in Ar-Manus’s mental network, they are still purely slaves to the mind’s will such is the corrupting demon influences enthralling them.

Demonic illithids, known more commonly as ithilaur, are completely immune to being subjugated by other elder brains or ulitharid and if an ithilaur were to come within range of either it is quite likely that that mind flayers under the elder brain’s control would be driven to insanity. Their greatest strength, their interconnected mind, would in this way become their greatest weakness for should an ithilaur infect one mind flayer it would infect them all simultaneously with there being little, if anything, the elder brain could do to stop it.

The mind flayers, now under the ithilaur's sway, would slay the elder brain they had been subsurvient to, gather up the remains of the elder brain and as many of their colony's resources as possible before seeking a way into the Abyss to join their new master. The corpse of the elder brain would be fed to the demonic elder brain of Ar-Manus while the new mind flayers under its control would begin to be altered over several decades of Abyssal mutation and deadly encounters into ithilaurs themselves.

A demonic ulitharid has not yet been spawned in the 664 Layer of the Abyss and no one is sure what will happen should such an event take place. Especially with the Dark Factorium acting as the largest known illithid colony on in the planes.

The Gith

Being mortal enemies of the illithids, it should come to no surprise that the Githyanki and Githzerai would have become interested in eliminating the demonic elder brain of Ar-Manus and its massive colony of mind flayers. However, many concerns were raised regarding how aware the Gith should be made aware of this threat.

Those deeply invested in the outcome of the Blood War have made it part of their mission to dissuade such information from reaching the Gith factions out of fear of the impact their interference could have. The two, warring sides of the race notwithstanding, they fear a potential full-on invasion of the Abyss to destroy Ar-Manus’s illithid colony could have several catastrophic ramifications should it come to pass.

First, it would indeed require an immense invasion force to deal with such a massive threat; the likes have which have never been attempted before into the Abyss. The effect the Abyss could have on the Gith being one concern but the other, on one far more terrifying, is the idea that a full-scale invasion of the Abyss to silence a demon lord may embolden the Nine Hells into an infernal invasion of their own, may inadvertantly unify the demon lords for either the purpose of defeating those who dare attempt such a bold endeavor against the demons of the Abyss, dissuade involvement by the Nine Hells, or both, and overall may just upheave the Blood War and break the stalemate.

Second, the vulnerable state this would leave the Astral Plane in particular is a thought very much worth considering especially if any other illithids could establish a foothold in the meantime. A mind flayer colony could very easily bloom in the Astral Plane if left unchecked and could very well become the start of the Illithid Empire's rebirth.

And thirdly, with the Dark Factorium’s defenses, how the ithilaurs are a different foe than what the Gith are used to facing, the demonic hordes of the Abyss which the Gith are absolutely not used to facing, and them walking straight into a demon lord’s stronghold all point that a zealous invasion by the Gith would ultimately end in failure anyway.

As such, efforts have been made to ensure that exposure to this knowledge is minimized to the Gith and those of that race who do learn of it are quickly intercepted and forced to consider the full ramifications of that knowledge reaching the rest of their people.

Ar-Manus lairs itself within a massive, ringed complex of metal fortresses, obsidian pillars, and admanatium spikes known as the Dark Factorium through which lava flows from freely through a streamlined and efficient adqueduct system from eight, massive supervolcanoes the encapsulate the epicenter of the Hadean Circle, the 664th Layer of the Abyss where Ar-Manus resides. The interconnected networks of symmetrical causeways, lava flows conduits and circuits, and strongholds littered with Ithilid architecture that make up the Dark Factorium are populated by a hierarchy of demons twisted by ithilid experimentation, demonic golems, and evidence of a large ithilid colony hidden deep within. What machinations the Demon Prince of Emptiness plots within this terrible place one cannot only speculate and could likely hardly even fathom.

Lair Actions

On initiative count 20 (losing initiative ties), Ar-Manus takes a lair action to cause one of the following effects; Ar-Manus can’t use the same effect two rounds in a row:

• One of the causeways connecting the various fortresses and other structures on the ring either splits and retracts into its base removing this crossing or extend out to create one. This movement happens over the course of three turns.
• Heated gas is sprayed into a room inside the lair through vents. Any creature in the gas is blinded and takes 5 (1d8) fire damage per turn. If the creature inhales the gas, they must succeed on a DC 16 Constituion saving throw or take 6 (1d10) poison damage and become poisoned. This poisoned status lasts until the creature is out of the gaseous cloud. The gaseous cloud is not breathable. This effect lasts until Ar-Manus uses another lair action.
• The force of gravity increases within a 15-foot cylinder indicated by a violet light in the area. Any creature that starts or enters this area must succeed on a DC 20 Strength saving throw of have their movement reduced by half and suffer disadvantage on all actions that require them to move their body. Ar-Manus is unaffected by this effect. This effect lasts until Ar-Manus uses another lair action.

Regional Effects

The region containing Ar-Manus’s lair is twisted by its magic and psychic presence, which creates one or more of the following effects:

• All creatures within 1 mile of the lair suffer the constant sensation of having their brain poked and prodded at as if something was probing their mind. This sensation gets more powerful the closer the creature gets to the epicenter of the lair.
• Golems within 1 mile of the lair will begin to disobey their masters or programming. Prolonged visitation - 1d10 days - to the region will result in the golem revolting entirely, potentially lashing out at former masters or associates. Additional time within the region will result in the golem coming under the thrall of Ar-Manus.
• If a humanoid spends at least 1 hour within 1 mile of the lair, that creature must succeed on a DC 18 Wisdom saving throw or descend into a madness determined by the Madness of Ar-Manus table. A creature that succeeds on this saving throw can't be affected by this regional affect again for 24 hours.

If Ar-Manus dies, these effects fade over the course of 1d10 days.

Madness of Ar-Manus

 If a creature goes mad in Ar-Manus's lair or within line of sight of the demon lord, roll on the Madness of Ar-Manus table to determine the nature of the madness, which is a character flaw that lasts until cured. See the Dungeon Master's Guide for more on madness.