This is my conversion of the Mercurial creature type from Doors to the Unknown, as well as the signature items designed for dealing with one. I opted to use Deva as a base, as it was pretty close to the original 2e implementation.

On one of the peaks of Mount Celestia is the realm of Beldaari, a land so deep into the plane of Lawful Good that it is almost beyond this pale cosmos. In this hyper-real forest, the beings of other planes seems lesser, barely real. Bathing in the light of their unnamed god of pure thought, the Mercurials exist in a state of contemplative bliss. While mostly similar to Devas, they have a strange ability to experience life from the bodies of other beings by shedding their own skin and bones. When they can be convinced to leave their home realm this ability to transcend their own form can make them profound allies to the Celestial forces, as they cannot be permanently harmed while they have left their flesh behind.

While most, beings of pure law and good, would only use this ability on the willing, and leave before doing harm, tales tell of one of their own that went mad. Lathuraz wished for a mighty weapon, had it granted by his god who could not believe betrayal was possible, and used it to cut his way free of his brethren, taking the forbidden portal into Sigil that opened on their land every 500 years. His brother, Zarulaz, trailing behind to try to stop him, he nonetheless wreaked havoc for centuries. When turned to evil, an unkillable, charismatic warlord who can steal the forms of his enemies can quickly amass a tremendous base of power. Finally, with the next blink cycle and the help of Zarulaz, Lathuraz was imprisoned behind one of the doors, awaiting heroes to gather the requisite tools to destroy him forever the next time the doors opened… or stand back as he began his conquest anew.


Medium celestial, lawful good

Armor Class 17 (natural armor)
Hit Points 136 (16d8 + 64)
Speed 30 ft.

18 (+4) 18 (+4) 18 (+4) 17 (+3) 20 (+5) 20 (+5)

Saving Throws Wis +9, Cha +9

Skills Insight +9, Perception +9

Damage Immunities bludgeoning, piercing and slashing damage from nonmagical weapons

Damage Resistances radiant

Condition Immunities charmed, exhaustion, frightened

Senses darkvision 120 ft., passive Perception 19

Languages all, telepathy 120 ft.

Challenge 10 (5900 XP)

Borrow Form. The mercurial may shed its skin and bones as an action. They are left in the space this action was performed as an object with AC 13 and hit points equal to what the mercurial had upon leaving them. While standing in the same space as its skin and bones, the mercurial may re-enter them as an action, setting its hit points to the average of its current hit points and those remaining in the object. While outside of its skin, the mercurial’s AC is reduced to 14 (unarmored).

While not in its own skin, the mercurial may attempt to “borrow” the form of another medium- or small-sized humanoid. The mercurial must successfully grapple the target, then attempt to borrow the form as an action on its turn. The target may make a Constitution saving throw (DC 17) to resist this possession. Upon failure, the mercurial disappears into the target’s form, and the target is incapacitated and loses control of its body. The mercurial now controls the body but doesn’t deprive the target of awareness.

While borrowing a form, the mercurial uses its normal statistics except it wears the armor and wields the weapons of the borrowed character. The borrowed character’s hit point total becomes temporary hit points for the mercurial.

While possessed, the host gains a level of exhaustion every X minutes (equal to the character’s Constitution score plus level or CR). The mercurial does not suffer the effects, but they will apply to the body when the possession ends. If the mercurial’s temporary hit points are reduced to 0 or six levels of exhaustion are accrued by the host, the host dies. The mercurial does not have to relinquish the form, but cannot re-borrow it upon leaving (and decay and damage will become apparent over time).

The possession lasts until the mercurial chooses to end it as a bonus action or the mercurial is forced out by an effect like the dispel evil and good spell. When the possession ends, the mercurial reappears in an unoccupied space within 5 feet of the body. The target is immune to this mercurial’s Borrow Form for 24 hours after succeeding on the saving throw or after the possession ends.

Innate Spellcasting. The mercurial’s spellcasting ability is Charisma (spell save DC 17). The mercurial can innately cast the following spells, but only while on its home plane, requiring verbal components:

At will: detect evil and good
1/day each: commune, wish

Magic Resistance. The mercurial has advantage on saving throws against spells and other magical effects.

Mercurial Weapons. The mercurial’s attacks are magical. When the mercurial hits with any weapon, the weapon deals an extra 2d8 psychic damage (included in the attack).

Regeneration. The mercurial regains 10 hit points at the start of its turns. It loses this ability if its skin and bones are reduced to 0 hp, either while worn or as an object (see Borrow Form). The mercurial only dies if it starts its turn with 0 hp while its regeneration is not active.


Multiattack. The mercurial makes two weapon attacks (either with wielded weapons or with its slams). One or both of these attacks can be used to attempt a grapple.

Longsword (or Other Host Weapon). Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 8 (1d8+4) slashing damage plus 9 (2d8) psychic damage.

Slam. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit:7 (1d6+4) bludgeoning damage plus 9 (2d8) psychic damage.

Mental Blast (Recharge 5-6). The mercurial magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 17 Intelligence saving throw or take 23 (4d8+5) psychic damage and be stunned for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Image of monster block

Anti-Lathuraz Items

Cloak of Trapping

Wondrous item, very rare
Keywords: Outlands, Confining, Cutting, Dark, Disjointed, Metallic

This distinctive black cloak with silvery star patterns can be used like a net to attack a creature with shapeshifting or possession abilities (and hits automatically if the creature is convinced to wear it voluntarily). Instead of being restrained, on a successful hit the creature is trapped in its current body and/or shape. Attempts to grapple such a target have advantage. While so trapped, the cloak cannot be damaged (attacks simply hit the captured target), and the target’s Strength check DC to escape is 18 (+1 per matched keyword).

It can be worn as a normal cloak by individuals without shapeshifting or possession abilities. It grants the wearer +1d4 on attack rolls to grapple a target. It can be removed quickly by such a bearer as part of the action to attempt to capture a target.

Rod of Mercury

Rod, unique
Keywords: Mount Celestia, Lawful, Good, Bright, Colorful, Metallic

This two-foot cylinder appears to be made of molten silver but is solid when touched. It tugs toward the nearest mercurial on the same plane, with increasing force if the target is close by (but never hard enough to pull free of a grip or move on its own). This allows the bearer to work out the direction and distance of nearby mercurials.

If there is more than one such creature on the same plane, the bearer may make a Charisma check to track a specific mercurial rather than the nearest. The bearer must have seen the target before and speak its name as a command word. The check DC varies based on the number of mercurials nearby and the distance to the intended target. It is generally equal to 15 +1 for every intervening mercurial but -1 for each matched keyword.

Manacles of Lesser Reality

Wondrous item, legendary
Keywords: Unknown Prime World, Confining, Disjointed, Invisible, Mystic, Smashing

This pair of iron bracers with sunburst designs on them have no obvious locking mechanism. While there is no visible chain, they are connected by an implacable force and cannot be separated by more than five feet before snapping back together. They can be attached automatically to the wrists of a willing or unconscious target as an action. If the target is active and unwilling, they must be applied as an action by an individual that has successfully grappled the target.

While worn by a target that would be affected by protection from evil and good, the manacles deactivate the target’s most powerful abilities and actions. In particular, innate spellcasting and attacks that produce energy are usually suppressed while the manacles are worn. If only one manacle is worn, the subject cannot use such abilities on the wearer of the other manacle, and that wearer gains the effects of protection from evil and good against the target. Further, the target cannot move the wearer of the other manacle by pulling against the invisible chain of force: the target simply becomes restrained upon reaching the end of the tether, and may only move in the direction of the other bearer. Any attempts to teleport the target fail if the other bearer objects to the relocation.

The being who applied the manacles may remove them as easily as using any other object, the manacles falling open at a touch. Otherwise, removing them requires a thieves’ tools check (at a DC equal to 20 + 1/matched keyword), on which the creature trapped has disadvantage. This DC drops by 5 if the one who attached the manacles is dead.

If willingly donned by a wearer that is not subject to the effect, the wearer can remove them at any time. They grant the wearer the benefits of the protection from evil and good spell while both bracers are worn, but prevent the wearer from casting spells of 4th level or higher or using supernatural class abilities granted at 7th level or higher. Further, the wearer can remove one manacle and attach it to another willing target or structure that the bracer can fit around (e.g., pole or rope) to make use of the force tether. In this case, neither target loses movement when at the extent of the tether, and physics determines movement.

Sword of Lathuraz

Weapon (longsword), rare
Keywords: Mount Celestia, Lawful, Good, Bright, Colorful, Metallic

You gain a +1 bonus to attack and damage rolls made with this magic weapon. The regeneration trait of a creature struck with this weapon doesn’t function at the start of its next turn (but functions on subsequent turns if not struck again by the sword). Further, a creature reduced to 0 hit points by this weapon that survives gains a permanent level of exhaustion (even if normally immune to exhaustion), and can gain additional permanent levels up to 3 if so “killed” multiple times, but at most one per day. This exhaustion manifests as permanent wounds from the weapon, and can only be removed by greater restoration or similarly powerful magic.

This longsword has an extremely wide-swept guard and appears to be made of silver and mithril that fade in brightness if the wielder is not lawful good. It was created in secret by the mercurial traitor Lathuraz, used to slaughter his way out of his home plane during his betrayal and fight off any of his kinsfolk that attempted to return him. It is said to have turned black as wrought iron when wielded by its creator after his escape.

The sword sheds light as a torch. If the bright keyword is matched, it sheds light as the daylight spell.

It is an ideal anchor point for visual illusions. Illusions that cannot normally move can be attached to it, moving as you move the item. If the colorful keyword is matched, illusions so attached cause the sword to deal bonus psychic damage on each hit equal to 1d4 + the spell level used to create the illusion (e.g., 1d4 for minor illusion or 1d4+3 for major image) until the illusion is disbelieved.

It resonates with nearby silver and mithril, granting advantage to detect hidden caches of these metals. If the metallic keyword is matched, attacks with the sword deal 1d6 bonus radiant damage on each hit against targets that are vulnerable to silver weapons (e.g., damage resistance overcome by silver or magic weapons).

The sword is uncomfortable to wield by characters that are not of lawful good alignment. If the lawful keyword is matched, chaotic wielders suffer disadvantage on attack rolls, and if the good keyword is matched, evil wielders suffer this disadvantage. If both keywords are matched, and the wielder is lawful good, spell slots expended to smite (or used to cast smite spells) with the sword are treated as one level higher.