The Warlock Insane
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Book ten of the Warlock Of Gramarye series by Christopher Stasheff, published in 1989.
Annotation:
"Until now, Rod Gallowglass could fight the numerous challenges of Gramarye with technology. But when an evil sorcerer uses a new drug to cloud his mind with horrific visions, Rod faces his most crucial test and Gramarye faces its ultimate challenge."