Shadow Beasts

Description
Shadow Beast has transparent and featureless humanoid form. Its eyes glow yellow like Dire Ghost's do. Its only form of combat is hitting with powerful claws. Despite what the lore says, this creature is not totally invisible. It only goes invisible briefly during the combat and reappears at slightly different position, making hitting it difficult. Avatar encountered only two of them during the Ultima Underworld: The Stygian Abyss. Shadow Beast haven't been seen since.

Lore
''There is little we can say about this entity, or family of entities - we obviously cannot see that which is invisible, so we do not know how many there are. The Shadow Beast is thought to be immensely powerful, more than a match for any single warrior. When attacked by unseen adversary, first look up (for a flying attacker) and down (lest a small worm or other foe is striking) - if none of these be the case, then suspect the Shadow Beast and begin to strike in all directions!''

- from "Memoirs of Sir Cabirus", Ultima Underworld I