Elistaria

Elistaria is the dark lady of Windemere, a very prominent member of the Oppression.

The Avatar had to meet her in Ultima V, to get the Black Badge. Elistaria asked for the password of the Oppression and after receiving it, gave the badge to the Avatar to officially welcome the hero in the Oppression. She also had managed to summon Daemons and undead to her bidding and was fond of these creatures.

She also told the Avatar to ask Thrud for a special item he had in store. (However, the item in question was a fake)

In Lazarus
Elistaria has been more fleshed out in the Lazarus version of the game. Here she's actively training dark mages for the Oppression, revealing herself to be a cruel and dominating woman. Her background is also much improved.