Demons also known as the "Umaiar", are spirits of old who turned over to the dark side. Apart from Morgoth, who was originally a Valar, most of the demons were corrupted Maiar (such as Sauron and the Balrogs). Corrupted Maiar were referred to by the Quenya word Umaiar.
- Sauron the eponymous Lord of the Rings, was a fallen Maia, the creator of the One Ring, and the most trusted lieutenant of his master Melkor.
- Nazgûl were the dreaded ring-bound servants of the later Dark Lord Sauron in Middle-earth throughout the Second and Third ages, and in the later years of the Third Age they dwelt in Minas Morgul and Dol Guldur. Famous are the two riders Witch-king of Angmar and Khamûl.
- Balrogs - spirits of the Maiar that were seduced and corrupted by Melkor to his service.
- Ungoliant was a primordial being in the shape of a gigantic spider. All giant spiders who lived through the ages were descended from her.