Aram-Dol, from Ultima VII Part Two
Within his lair in the caves surrounding the Keep, lay powerful magical artifacts and great treasures, along with one of the Orbs required by the Avatar for completion of the Silver Seed quest in Ultima VII Part Two
Another item of particular note which can be liberated from Aram-Dol is Erinon's Axe.
While powerful, Aram-Dol can be dispatched using a Glass Sword.
Aram-Dol created the Arachnians.
This is not so much a creature as it is a being, but I consider it worthy of note here. All of the folk I have spoken to in the keep are rather retiscent about the liche, but I have compiled the following information: The liche was here before the keep was established. It is undead; once alive, it entered a state of "unliving" which heightened its power but nullified what humanity it may have had when it was alive. It can command undead and hurl the most devastating of spells.
Know, adventurer, that to proceed onward thou must pass three puzzles. At the first test, throw two down. At the second test: throw three down to get eight. At the third test: throw four down to get twelve. If the levers are not in the correct position when the button is depressed, thou wilt surely be destroyed. Aram-Dol