Lilith: A Snake in the Grass is the first book in the Four Lords of the Diamond series by author Jack L. Chalker. First published as a paperback in 1981.
Somewhere, from among the four human settled worlds of the Warden Diamond, hostile aliens were spying on Earth. But no agent could be sent to investigate and report back. All trips to the Warden worlds were one-way. There a microscopic symbiont called the warden bug invaded all life forms, after which life became impossible outside the Warden system.
That called for extraordinary means, of course. One agent was chosen, then four men were stripped of their minds and personalities, and his was imposed upon them. Hooked up properly, he could then receive their report, without ever leaving safe territory.
On Lilith people with strong wills manifest the ability to sense and communicate directly with the warden bug. These sorcerers or Sorcs have tremendous power and makeup the political spetrum of the planet. Cal Tremon's mission was Lilith. Assassinate the reining Lord and find evidence of an Alien presence.