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 hitting it difficult. The Avatar encountered only two of them in the first Ultima Underworld. Shadow Beasts haven't been seen since, possibly destroyed by the eruption of the Abyss.