Before Morgoth was banished from Arda he had started to meld himself with the world itself. After his fall this Morgothic element, known as the Mordo ("Stain, Shadow, Grotesque") remained within Arda herself. This was the essential idea posited in the book "Morgoth's Ring" (written by J.R.R. Tolkien and edited by his son Christopher Tolkien).
Magicians who drew upon the Mordo were known as Black or Dark Magicians, Shadow-mages, (Evil) Sorcerers, Black Witches, and similar terms. The secret knowledge about how to gain power from the Mordo was known as Morgul (S."Black Knowledge") or Guldur ("Sorcery"), Felu ("Evil") or Black Magic, or Shadow-magic.