Demons were evil spirits that ranked among the most powerful of Morgoth's servants. Among the mightiest demons were the Balrogs, who were fiery Maiar seduced to Melkor's service in the beginning of the world.
Portrayal in adaptations
1982-97: Middle-earth Role Playing:
- Demons are portrayed as to exist in several types: Balrogs, Black Demons, Lassaraukar ("Leaf-demons"), Vampires, Demon-trolls, etc.
- A distinction is made between "animate demons" and "place-demons". The former have a body, and entail such creatures as lesser and greater Balrogs, Helegrogs ("demons of ice") and Dindair ("silent shadow", demons of shadow). Place-demons (bandúrhoth, "people of the dark prison") are incorporeal and "cannot be physically battled", an example being Caradhras the Cruel.
2007-: The Lord of the Rings Online:
- The Regmyl (sing. Rogmul) are fire-demon spirits, called forth by the will of a Balrog.
- In both games, a Graug is a giant Troll-like monster. Across both games, five different forms of Graugs are seen: Common, Fire, Frost, Poison and the even larger Great White Graug hunted by Torvin. All these are very different from the Balrog also seen in Middle-earth: Shadow of War, whose design is very similar to the Balrog from the Peter Jackson films.  
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Valaquenta: Of the Enemies"
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Quenta Silmarillion: Of the Flight of the Noldor", "...countless became the hosts of [Morgoth's] beasts and his demons"
- J.R.R. Tolkien, "Words, Phrases and Passages in Various Tongues in The Lord of the Rings", in Parma Eldalamberon XVII (edited by Christopher Gilson), p. 48
- J.R.R. Tolkien, "I-Lam na-Ngoldathon: The Grammar and Lexicon of the Gnomish Tongue", in Parma Eldalamberon XI (edited by Christopher Gilson, Arden R. Smith, and Patrick H. Wynne), p. 42
- Ruth Sochard Pitt, Jeff O'Hare, Peter C. Fenlon, Jr. (1994), Creatures of Middle-earth (2nd edition) (#2012), pp. 84-91
- Wesley J. Frank, et al. (1997), Arnor: The Land (#2023)
- Scott Bennie, Mike Mearls, Steve Miller, Aaron Rosenberg, Chris Seeman, Owen Seyler, and George Strayton (2003), Fell Beasts and Wondrous Magic, pp. 16-20
- Monster:Rogmul at Lorebook.lotro.com (accessed 5 March 2011)