Red Magic

Red Magic, by Jean Rabe, is the third book of the loosely-related The Harpers series.

"The Red Wizards rule Thay, perhaps the most wicked and corrupt land in all the Forgotten Realms. While the evil mage, Zulkir Maligor, builds a net of treachery and deceit, the Harpers seek to uncover his vile schemes. A small band is sent by the Harpers to pose as slaves and slavers to infiltrate the malevolent land. Once inside Thay, they face the undead minions of the Red Wizards and a future far worse than death: becoming the wizards' soulless puppets!"