Serving as an aide to Ambassador Shras, Thelev's mission was to destroy the USS Enterprise, which was responsible for transporting delegates to the Babel Conference. The purpose was to instigate mutual suspicion and possibly interplanetary war. Had war broken out, the Orions would have cleaned up supplying dilithium to both sides while continuing to raid Coridanite.
He murdered the Tellarite ambassador Gav by breaking his neck and later tried to murder James T. Kirk with a knife. He managed to stab Kirk but was captured and put in the brig. When he tried to escape he was stunned with a phaser and fell so that one of his antennae broke off and revealed a hidden device he had used to communicate with his ship. When Enterprise damaged his ship, it self-destructed and Thelev took a poison that killed him. (TOS: "Journey to Babel")
The TOS timeline in the novel Federation picks up immediately after "Journey to Babel" and states that the knife Thelev used against Kirk was poisoned. Leonard McCoy treated the wound but it continued to pain Kirk for decades afterwards.