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!"

Summary
Thay is ruled by a council of eight specialized archmages (Zulkirs), one for each school. Maligor, the Zulkir of transmutation, plots to take control of Thay by himself, and destroy his nemesis, Szass Tam.

His plan can be said to have three stages:
 * 1) Take control of Thays gold production, using his loyal spirit naga ( or Shinomen naga )assistant.
 * 2) Destroy Szass Tam.
 * 3) Invade Aglarond, and conquer the world.

This power battle has raised the attention of the Harpers, who send three of their agents to find a way to stop Maligor's plan: a big city mage from the court of Aglarond, a druid who has never even been to a town, and his slightly more civilized centaur friend who is a fugitive from Thay.