The Light in Exile — Cheryl J. Franklin

THE ADRAKI MENACE--
They roved the spaceways, calling no world home, though they conquered and then destroyed many a promising planet. They were the Adraki, last survivors of an alien race. Some said they were bent on bringing the rest of the universe down with them. Others claimed they sought a treasure they had foolishly lost long ages ago.

But now a man named Caragen, the most powerful individual in Network space, sought to ally with the Adraki, and he would use any means to achieve this goal--even if it meant sacrificing everyone on Siatha. For the Adraki seemed to have a sudden interest in this backward, non-tech planet, and Caragen was determined to find out why. His weapon of choice was Marrach, who though listed as Human had once been something else, and who had never failed in any mission.

But Siatha, home to the legendary Healers, was a challenge greater than any Marrach had ever faced--the challenge of a power as alien and uncontrollable as the dreaded Adraki themselves....