Title: Hunt the Heavens (Shadow Warrior Bk 2)
First published: July 31, 1996
Description: From the back cover:

END OF THE LINE

Joshua Wolfe: Bonty hunter, Federation Intelligence agent.  Former friend, then prisoner, of the horrific alien race called the Al'ar.  Master of the arcane Al'ar killing arts.  And now Federation renegade, searching for Al'ar secrets in the rubble of an old war and the detritus of human lives.

No one wanted him to learn those secrets.  Not the Federation, which had hired him, then made him a wanted man.  Not the deadly Chitet cult, who had their own interests in the Al'ar.  And not the Al'ar themselves, whose shadows remained in the form of technological wonders of mystery and violence.

His search led him from a mothballed fleet to a private moonlet, to the capital world of the very cult bent on destroying him, to a lethal world of robot protectors... and, finally, to an enemy more terrible than anything mankind had yet dreamed of.

Wolfe's only friend was Taen, the last Al'ar.

Or was he the last?