Middle Earth: Sunset of the Fourth Age
“Dark” elves, or Moriquendi, are those elves that either refused the summons of the Valar (the Avari), or else fell away from the journey west to Aman (the Úmanyar). They therefore never saw the light of the Two Trees before the Darkening of Valinor which ushered in the First Age.
In particular, this term within the campaign refers to the Avari (“the Unwilling”), who never traveled west at the summons of the Valar.
They live in the far East of Arda, beyond Mordor and the Eastern Marches of Rhovanion, even east of Rhun. Perhaps as far east as the Sea of Helcar, on whose shores the Elves first awoke in ancient times.
They speak a dialect of Elvish that is much removed from Sindarin, and almost as much from Quenya (“high” modern Elvish), as it branched off when the other elves journeyed west.