The fell beast, D, is a type of monster that appears in SpliceHack and EvilHack. It is a strong, thick-skinned and dragon-like carnivore that acts as the steed of Nazguls - it is capable of flight, can see invisible and has infravision.
A fell beast has two claw attacks and a bite attack.
Fell beasts are not randomly generated, and are always created hostile.
A randomly generated Nazgul will have a fell beast serving as their steed.
It was a winged creature: if bird, then greater than all other
birds, and it was naked, and neither quill nor feather did it
bear, and its vast pinions were as webs of hide between horned
fingers; and it stank. A creature of an older world maybe it was...
[ J.R.R. Tolkien, The Return of the King,
"The Battle of the Pelennor Fields" ]