The Warlock Enraged by Christopher Stasheff Warlock #4 1985 Ace
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.