Don't Fear the Reaper

From the introduction: At one time, Siegfried Mercy was the most feared and cruel necromancer of a generation, responsible for a hundred thousand ruined lives. Vanished into obscurity when a ragtag group of heroes defeated him and broke his power, he has never faced justice for his crimes. You have been hired by the Director of Royal Intelligence to locate and retrieve him in the Adamantine Archipelago, where he has allegedly resurfaced.