Illustration Copyright 2008 Wizards of the Coast.

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Every time you kill a demon, its soul is sent screaming back to the Abyss, where it will eventually reincarnate as a new demon. However, sometimes the process goes wrong, and a bunch of demon souls bump into each other on the way to the Abyss and fuse into a big clump. This katamari of evil can’t reincarnate properly, and instead rises as an undead demon. And this is how immoliths are made!

Immoliths look like pillars of fire topped by a demonic skull. Many different skeletal limbs emerge from their flames, likely belonging to the original demons that make it up. As undead demons, their hate of the living is double-plus-big, and they’re as likely to be found alongside powerful undead like liches and vampires as they are with other demons. They’re only statted up in the Monster Manual.

Immoliths are Large Elemental Magical Beasts (demon, fire, undead), and Level 15 Controllers with 153 HP and defenses a point or two lower than what you’d expect. Interestingly, they lack darkvision, but I imagine that’s not needed when you’re walking bonfire.

Immoliths are immune to fire, disease and poison. They have Variable Resistance 15 (switchable once per encounter), and Radiant Vulnerability 10. The intense heat emanating from their bodies works as an Aura 1 that deals 10 fire damage to anyone starting their turns within it.

Their basic attack is a Reach 4 claw that does both immediate and ongoing fire damage. Instead of that they can use a Fiery Grab against Large or smaller targets. This combines the claw attack with a grab that pulls the victim adjacent to the immolith (and into its aura). The demon can grab up to five creatures this way, and while they’re grabbed they lose any resistance they have to fire. Did you think you were safe just because you were a tiefling, fire genasi or fire sorcerer?

As a minor action, the immolith can inflict a Deathfire Curse on someone as a Ranged 10 attack that targets Will. On a hit the target is slowed (save ends), and after they save from that they’ll take ongoing 5 fire damage (save ends).

Also as a minor action, though only once per round, it can restore 5 HP to all undead in a Close Burst 5, including itself.

It’s not inconceivable that one of these could have the entire party grabbed and taking ongoing fire damage from their aura. People who try to run would be slowed by the Deathfire Curse, and likely by the immolith’s buddies.

The sample encounter is level 14: one immolith, 2 vrocks, 4 mezzodemons, and 6 horde ghouls. Since it appears both mezzodemons and vrocks have means to limit a character’s mobility, my analysis above checks out.