Grath was a Scarran interrogator who abducted John Crichton on a commerce planet and brought him to an underground interrogation chamber. There, he was able to induce a mental hallucination of Earth to torture Crichton with.
Grath was, however, unaware of the neural clone of Scorpius in Crichton's mind. The clone shut down Crichton's brain activity -- leading Grath to believe the human was dead. Crichton used this deception to put his pulse pistol in the Scarran's mouth and overload it -- blowing off Grath's head and much of his torso.
Grath is derived from Farscape.