Erethian, from Ultima VII
Erethian discovered the submerged Isle of Fire a little over 200 years before Ultima VII, having scouted it out in his time there. Around this time he successfully bound Arcadion to his will in a mirror, much to the daemon's displeasure. He then dedicated his life to the study of the Dark Core -- the hard disk of Exodus. He eventually found a way to raise the Isle of Fire back to the surface, thus alerting Lord British and the Avatar to his doings.
At first, Erethian was helpful when the Avatar arrived on the Isle of Fire, giving much information and helping in the construction of The Black Sword. However, when he found out the Avatar's intent was to banish the core, he became hostile. When he tried to stop the Avatar from doing it anyway, he blundered his spell and it backfired, killing him on the spot.