Dracothraxus, from Ultima VII
Dracothraxus had somehow managed to enter the submerged Isle of Fire, surprising Erethian and stealing his Ether Gem in the process. She seemed to know Arcadion by that time already and stole the Gem mainly to annoy him, as he thought it to be his key to freedom. But fleeing into the Test of Courage made her part of the test, trapping her there.
The Avatar met her in Ultima VII, but killing her was futile, as the test had made her immortal. However, as a reward for the effort she gave the Ether Gem to the Avatar. She had no idea that the Avatar would use it to create The Black Sword. Returning with it, the Avatar used it to slay Dracothraxus with Arcadion's powers. But she was relieved that it was finally over and her spirit was free to roam again. She congratulated the Avatar on this success, then vanished into the void.