Formerly the magical advisor to Lord Yarvish, Cullen’s investigations into the arcane led him down the path to necromancy. Eventually powerful enough to curse and enslave all the inhabitants of the King’s Seat, as well as cause a blight in the land, he escaped to the mountains to complete his ritual.
The reversal of Kelemvor’s spell to keep undeath in check brought the man to the attention of the god and caused him to summon avatars, unlikely adventurers whose origins dotted the globe. Though they fell prey to the twisted inhabitants of the Seat, they were subsequently resurrected and eventually completed his mission.
The last the heroes saw of Cullen was him being pulled into the sky by Kelemvor after the battle at the mountain cathedral, presumably to answer for the unnatural rites he was performing.