Jack Brundle

Jack Brundle is/was a powerful sorcerer, now a spirit trapped inside his ring after a failed attempt to become a lich.

Born a Yentish peasant, Brundle rejected the magic-shunning ways of his community to become one of the great sorcerers of his age, siding with the necromancers against the yentish kings. After the downfall of the necromancers, he remained at large as an outlaw for some 30 years before being slain by King Oswalt I

He's sort of a boogeyman figure in much of Yent.

He created the brundleplane and all within it.

He's currently trapped inside the mind of Fen Widefield