The Bodak is a cross between undead and demon, introduced in the 32-page insert in 1982 adventure S4: The Lost Caverns of Tsojcanth. They "are evil humans changed into monsters by exposure to the demonic forces and substances of the Abyss".
Appearance and Abilities
They're mutated shambling (MV 6") mummified corpses. Smokey-grey hairless skin, warped skull, milky white eyes. Hermaphroditic, for what that's worth.
They take damage in sunlight, and (like demons) against coldwrought iron. They are most feared for their death-stare: within 30', if you don't make your save v. death, guess what.
(All this implies that if you meet your end gazing at a Bodak while in its new Abyssal home, you're like to become one yourself. So stay away from the Abyss!)
Bodaks are sentient... sort of. They "speak" the language they had in their human form, but it is unlikely you'd care to hear what they have to say. They also speak the Abyssal languages.
On their home planes, Bodaks are summoned (back) by demonologists. If they're smarter than the demonologist (in INT; we can assume the demonologist a cretin in WIS) they can control him.
How to Make a Bodak
The Bodak entered the second Monster Manual and also Manual of the Planes, as made from evil visitors to the wrong Abyssal layers. But this didn't catch on, being restrictive.
Planescape figured out that, rather than outsiders as such, bodaks fill that niche in the Outer Planes that undead would in the Prime Material. It would stand to reason that the undead are souls who don't reach their afterlife. Inasmuch as the Outer Planes are the afterlife, logically these planes cannot have undead. (Although some foul gods might lock their damned into the form of undead.) Monte Cook wrote a particularly horrifying and poignant bodak for the Hellbound: The Blood War adventures.
|The inhabitants of the Planes of Planescape|
|Upper Planes:||Aasimon - Angels - Animal Lords - Archons - Asuras - Eladrin - Guardinals - Lillend|
|Middle Planes:||Formians - Githzerai - Inevitables - Marut - Modrons - Rilmani - Slaadi - Kamerel|
|Lower Planes:||Alu-Fiends - Baatezu - Bladelings - Cambions - Demodands - Erinyes - Hags |
Hordlings - Imps - Kytons - Loumaras - Marilith - Obyriths - Succubi - Tanar'ri - Yugoloth
|Transitive Planes:||Astral Dreadnought - Githyanki|
|Inner Planes:||Azers - Elementals - Genies - Grues - Mephit - Salamanders - Sylphs|
|Sigil:||Dabus - Cranium Rats|
|High-ups:||Archangels - Archdevils - Archfey - Archomentals - Demon Princes|