It has been several, several years since I read this novel, but I remember I loved it and it earned a special spot on my bookshelf. Let's see if I can recall more by telling you what the back cover says:
THE ADRAKI MENACE - They roved the spaceways, calling no world home, though they'd 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 sough a treasure they had foolishly lost 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 acheve 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 figure out why. His weapon of choice was Marrach, who though listed as Human had once been something else, and who has 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 Araki themselves...."
I remember being pleasantly surprised by Franklin's great skill as a writer, and that the story was more than entertaining, perhaps award worthy.
It looks like there are a few other books set in the universe since I read this one.