Pelias is a powerful wizard. He speaks Kothic, so it's likely that he's a Kothian. He's the rival of Tsotha-lanti. Tsotha was able to capture Pelias after he'd been drinking too much wine, and kept him prisoner in the Halls of Horror for ten years, before Conan rescued him. While in the Halls, Pelias was held by the plant Yothga which dulled his mind, preventing him from breaking free.
Pelias is able to raise the dead, and, by animating the dead guard Shukeli and commanding him to open the gates out of the Halls, he and Conan escaped.