Garvos was a mortal who, along with others, had the misfortune to enter the Realm of Lies, the domain of Belial in the Burning Hells. He suffered the realm's misdirections, Belial's laughter ringing in his ears the entire time.
Eventually he (was) returned to the mortal realm, by which time he was possessed by a demon. Holy magic was channeled into him, spurring him to give an account of his experiences which was recorded. It is said that after the testimony was given, the demon erupted from Garvos's body, killing him instantly.