Black Demons were lesser Maiar who had allied themselves with Morgoth. Their forms varied infinitely, as did their abilities. They could sometimes be persuaded to aid Men or Elves in evil deeds (or even in good ones, if heavily constrained by a Good power of great might), but unless the evil-doer requesting this service had protected himself or herself carefully and laid the strongest constraints on the Demon, little good would come of such bargains in the end. Demons often partook of some of the characteristics of natural forces: Balrogs as Spirits of Fire were a very good example. This was, no doubt, because of Eru's original plans for these spirits as directors and guardians of these natural forces.
Black Demons of note or renown[]
Carrog Drul Chaurka Fankil Gilthrang Kuruki Lairathin Langon Máugarth Moria doom spirit Morthrog Príclís Slyardach Tengu Tredeinos Tselakus Ûpo Zaken