"Yu-go-loth" or "Yu-go-loth"
In AD&D cosmology, Yugoloths are the paramortals sometimes called daemons by mortals. In any case, they are the incarnations of the neutral evil alignment. Yugoloths are found mostly on the planes of Acheron, Gehenna, the Gray Waste, and Carceri. Theoretically, they originated in the Gray Waste, since it is the plane of neutral evil, but the origin stories of all paramortal races are fairly sketchy, especially when the given paramortals are famous for their lies.
The term "daemon" is rarely used in reference to anything other than a Yugoloth. Nevertheless, "Yugoloth" is the term the Yugoloths use for themselves, and any mortal powerful enough to attract their attention has probably learned enough about them to know it.
The known types of Yugoloth are:
- Arcanaloth (being the snappily-dressed jackal-headded record-keepers of the Yugoloths)
- Nycaloth (being 9' gargoyles who fight with nasty double-bladed axes)
- Ultroloth (being the mysterious, robed, faceless masters of the Yugoloths)
Yugoloths all share the following abilities:
- Summon Yugoloths (number and type vary)
- Immunity to acids and poison and fire
- Resistant to gas attacks
- alter self, animate dead, cause disease, charm person, improved phantasmal force, produce flame, and teleport without error at will
And most have a handful of other abilities besides.
In appearance, Yugoloths tend to be humanoid, although they show more variation than the lawful Baatezu. Yugoloths tend to exude a sinister feeling. Good adjectives might be "mysterious," "shadowy," and, of course, "evil." Yugoloths cannot breed with mortals at all, nor do they have any desire to. Some legends claim that Yugoloths cannot reproduce at all, which explains their carefully honed cunning and tact. What is more likely is that Yugoloths are formed either from the souls of particularly evil mortals, whose souls form the larva that infest the Lower Planes, or that they are formed from the evil energies of Lower Planes themselves. Another theory, that they are formed from rogue Tanar'ri or Baatezu who swear allegiance to the Yugoloth cause, has never been substantiated.
Yugoloths are famous for their claim that they are the ultimate evil, untainted by law or chaos. They are cold, long-term plotters and mercenaries, and tend to show up only when needed or collecting for previous jobs.
Yugoloths act as mercenaries for both sides in the Blood War between the Baatezu and Tanar'ri, selling to whichever side can offer the best deal at the moment, though they are notorious for betraying their buyers.
Bonus politics of role-playing games
In first-edition AD&D, the word "Yugoloth" did not exist, and the creatures were simply called "daemons." However, thanks to the combined legal efforts of philistines, by the time second-edition came out, the more ambiguous term "Yugoloth" was created. By 2000, when 3e was published, the D&D franchise had enough money, lawyers, and giant corporate owners that the game was allowed to exercise its 1st Amendment rights and put these beasties back in the "D" section.
At least, in theory. Unlike "Baatezu" for "devil," and "Tanar'ri" for "demon," most gamers actually liked the term "Yugoloth" better than "daemon." Yugoloths did not make a single appearance in 3e Monsterous Manual, although there are reports that they will return as part of the new Manual of the Planes, and may very well be filed under "Y." The question remains unanswered.