Enemy

A term for either of the Dark Lords

A common term for either Morgoth or Sauron, or their forces.

Morgoth	The original Enemy was the dark Vala Melkor. After his murder of Finwë and theft of the Silmarils, he was named Morgoth, 'Black Enemy', by Finwë's son Fëanor, and was known by that name ever afterwards.
Sauron	Sauron was a servant of the first Enemy during the Elder Days, but after the fall of his master Morgoth, he took on the mantle of Dark Lord and Enemy of Elves and Men. The Free Peoples were reluctant to speak his true name, and thus he was often referred to simply as 'the Enemy'.