The Warlock Enraged
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Book five in the Warlock Of Gramarye series by Christopher Stasheff, published in 1981.
From the Publisher:
"A family crisis weakens the Gallowglass family's collective psychic abilities. The power of warlock Rod Gallowglass comes from a mind-link with the evil Lord Kern, Rod's other self in an alternate universe. And before they can take on Alfar's wizards, the Gallowglass family has to find a way to stop the madman who wants to possess Rod's soul -- from within."