The Sagittarius Whorl. Book Three of the Rampart Worlds Trilogy

Two centuries into the future, the Hundred Concerns, a group of powerful corporations that dominate galactic commerce, have pressured the Commonwealth of Humans into signing a pact with the Haluk, a conquering alien race with nefarious designs. Among the few people who recognize the malevolent intent of the aliens is hotheaded maverick Helly Frost. To prove that the Haluk have created demiclones--genetically engineered individuals who are perfect human replicas--Helly travels to the Sagittarius Whorl, a fearsome region of the galaxy hostile to every form of life.
But he must find crucial pieces of evidence that will expose the Haluk plot. Instead, he discovers something far darker than he had ever imagined.