1981’s1 Lilith: A Snake in the Grass is the first volume of Jack L. Chalker’s The Four Lords of the Diamond quartet.
The human Confederacy utterly dominates its region of the Milky Way. All known alien intelligences have been either subjugated to serve human needs, or where that was not possible, exterminated. Unbeknownst to the Confederacy, there are rivals. The first inkling the Confederacy gets that a new player has entered the game comes in the form of a failed infiltration of a defense facility by a sophisticated android double of a Confederacy functionary.
The android eludes capture. The Confederacy lets it flee in a stolen starship (at the cost of some expendable red-shirt lives), hoping that the android will lead humans to the alien home world. The mysterious rivals have technology equal to or perhaps superior to that wielded by humanity. The fact they have not fallen on humanity zap-guns blazing suggests that the aliens are not certain they would win, even with surprise on their side. If the Confederacy can find the aliens, they might be able to crush them.
The android does not flee to the alien empire, wherever it may be. Instead it leads the navy to the Warden Diamond, human worlds apparently in league with the aliens. Human worlds from which escape is impossible!
Dispatching an agent to the Warden Diamond will require a special agent and some extraordinary measures.