Brain Creatures

Basically these creatures are huge, flying brains with eyes. They attack their opponents with their psychic powers, literally tearing them apart. While physically weak, they are nonetheless strong opponents.

In Ultima Underworld II, these creatures were found in the Void. Two more used their magic on Killorn Keep to keep it flying. When the Avatar destroyed them, the Keep crashed to the ground, killing all inhabitants.

Trivia

 * Crashing the Keep is entirely optional.