The noble Gryph-Hounds have fought beside men and women of Sigmar for thousands of years.  Those that accompanied the General and were trapped outside Azyr were forever changed.  Beset by foes on all sides, the General made them stronger, awakening old blood within them.  They fed on the necrotic bats of Shyish, becoming something more.  Taking to the skies once more, and feeding on the blood of the living, hunters of darkness.

Counts as fell bats!