Vampires in the Return of the King

[The Dire Wolf]

There is one sentence about vampires in the troop list from Mordor; does anyone know any details about that?

What I'm looking for is if anyone has seen references to vampires in any other of the books? I know the Hobbit and LOTR do not... Not as familiar with the others.

Anyone ever see any? This seems a very odd thing he just mentions once...


Thanks!