Shadow Beasts

The shadow beast has a transparent, featureless humanoid form with glowing, yellow eyes much like those of a Dire Ghost. In combat it lashes out with its powerful claws.

Despite its description below, this creature is not totally invisible; it becomes so briefly during the combat, reappearing at a slightly different position, making it a challenging target. The Avatar encountered only two of them in the first Ultima Underworld. Shadow beasts haven't been seen since the eruption of the Abyss.