Molydeus

From Mordenkainen's Tome of Foes (2018):

The most ruthless and dangerous of demons — more feared than the dreaded balor — the molydeus speaks with the authority of the demon lord it serves as it enforces its master’s will. Standing some 12 feet tall, a molydeus has a red-skinned, humanoid body and two heads — one that of a slavering wolf and the other that of a serpent with dripping fangs perched atop a long neck. Molydei might guard their masters’ possessions, roam the battlefields of the Abyss to ensure the loyalty of troops, or bring swift death to their enemies.

Branded and Bound. When a demon earns the attention of a demon lord through ferocity, cunning, or an act of surprising devotion, the demon lord might reward such service by snatching up the fiend and subjecting it to excruciating torments to remake it into a molydeus.

Voice of the Master. A demon lord has a direct link to its molydeus and uses the serpent head to communicate its wishes. A molydeus is, therefore, said to utter its master’s will, commanding other demons to carry out orders and using violence to ensure they obey. A molydeus must constantly be ready for the scrutiny of its master, for the demon lord can decide at any moment to observe the molydeus through the serpent. Thus, there is no room for treachery in a molydeus.

Special Weapon. As part of a demon lord’s trust in its molydeus, it bestows a powerful weapon upon the guardian demon. The demon lord fashions the weapon from a portion of the fiend’s essence, so that the demon and its weapon are forever bound. If the molydeus dies, the weapon dissolves into a pool of foul-smelling slime. It’s possible to steal such a weapon, but a molydeus deprived of its weapon will stop at nothing to regain it.

The weapon a molydeus wields reflects the nature of its master. Those that serve Baphomet carry a glaive; Demogorgon, a whip; Fraz-Urb’luu, a battleaxe; Graz’zt, a greatsword; Orcus, a morningstar; and Yeenoghu, a flail. The weapon’s form doesn’t affect its capabilities.

Dark Guardians. One of the chief tasks of any molydeus is to help protect its master’s amulet — the most prized possession of any demon lord. Each of these dangerous relics allows a demon lord to return to life in the Abyss if the unthinkable occurs and its abyssal form is destroyed. As useful as these amulets are, they are also liabilities — because, armed with an amulet, a creature can coerce the demon lord to which it belongs into doing its bidding, or can strand it in the Abyss if the amulet is destroyed.

From Planescape: Monstrous Compendium Appendix I (1994):

The only guardian tanar’ri, the molydeus enforces the war effort as a sort of political officer.

Molydei have a form of ESP that lets them communicate with intelligent creatures and read the thoughts of others.

The molydei are the greatest enigma in the Abyss. These powerful police wander the layers of the Abyss and search for true tanar’ri that stray from the cause of the Blood War. They report directly to the balors, but even balors are not above reproach, and the molydei would turn against one that strays.

By enforcing the loyalty of the true tanar’ri, the molydei play an important role in the Blood War. These creatures exist only to serve the cause. They have no loyalty towards any tanar’ri and will try to destroy any of them at the slightest sign of infidelity. They do not enforce their doctrine on nontrue tanar’ri, for they assume that these are all disloyal by nature, and that only constant threats and punishments keep them in line.

Combat: A molydeus is never surprised. It attacks fearlessly and seldom retreats. Its enchanted axe inflicts 2d10 damage per hit and is +5 to both attack and damage rolls. The axe has the powers of a vorpal weapon and a dancing sword.

Molydei also attack with both heads. The dog head inflicts 2d6 damage; the snake head does ld6 damage and injects a powerful venom (save vs. poison or transform into a manes in 1d6 turns). A neutralize poison spell followed by remove curse eliminates the poison. Once transformed, the victim is beyond restoration, short of divine intervention or a very carefully worded wish.

In addition to those available to all tanar’ri, a molydeus has the following spell-like abilities: affect normal fires, animate object, blindness, charm person or mammal, command, Evard’s black tentacles, fear, improved invisibility, know alignment, lightning bolt (7 times per day), polymorph other, sleep, suggestion, true seeing (always active), and vampiric touch. Molydei can also gate in 1 molydeus, 1-2 chasme, or 1-4 babau once per hour with a 35% chance of success.

Molydei are immune to damage by most normal or magical weapons. Only cold-wrought iron weapons and magical spells can affect these creatures.

When a molydeus dies, its axe disappears. The only way to get this powerful weapon is to take it from a living molydeus. A molydeus does not rest until it recovers its weapon, stalking the thief day and night without end until the axe is recovered and the thief horribly killed.