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 Christoper Tolkien].
Magicians who drew upon the Mordo were known as Black Magicians, Dark or Shadow Mages,evil Sorcerers, Black Witches and similar terms.The secret knowledge how to gain power from the Mordo was known as Morgûl (S."Black Knowledge") or Guldur ("Sorcery"), Felu ("Evil") or Black or Shadow Magic.