This Darkening Universe, Paperback/Lloyd Jr. Biggle

Sweeping through the Small Magellanic Cloud, leaving a trail of ravished worlds and destroyed civilization, is the Udef -- the Unidentified Death Force. The best scientific minds of two galaxies can make nothing of it. The Udef is invisible. No scientific instrument can detect its presence. The only marks of its passing are the screams of its victims and the piles of dead left in its wake. Momentarily safe on the world of Montura, Jan Darzek's strategy against the Udeffalters. He and his assistant Effie have been ordered there by a supreme computer -- that isn't telling why. There is nothing to do on Montura but frantically wheel and deal in its spectacular interworld trading market. Supreme has also sent a perplexed Dr. Malina Darr from Earth, whose apparent purpose is to cure an unknown disease in an alien life form no one has ever seen ... The three of them must content with a mad mixture of Montura's mysterious race of natives, the melange of self-serving life forms in the marketplace, and the Kloatraz -- the universe's strangest "living" computer -- while the Udef comes ever closer to wiping out every intelligent life form in the universe. Somehow it must be stopped before no one is left to fight it.