|“||Agramon takes the form of whatever most terrifies you. When it is done feeding on your terror, it kills you, presuming you are still alive at that point. Most men — and woman — die of fear before that.||”|
Agramon was summoned by a young warlock named Elias for Valentine Morgenstern in September 2007. Upon being summoned, Agramon managed to break free from the pentagram because Elias' greatest fear was of any demon he summoned on a job escaping its pentagram. Agramon then proceeded to kill Elias and was put under Valentine's control through the Mortal Cup.
Agramon and Valentine later went to the Silent City to steal the Mortal Sword. Even the Silent Brothers present were not immune to his powers, and the Brothers present in the Silent City at the time of the attack were scared to the point of death. Jace, who was being kept in the underground cells, grew scared of the sounds the Silent Brothers made and the aura given off by Agramon's presence. Agramon revealed himself to Jace, who was imprisoned in one of the Silent City's holding cells, and knocked him unconscious.
Aboard Valentine's ship, Agramon showed Jace his greatest fear: a gravely injured Clary seemingly dying in his arms, after which Agramon again knocked Jace unconscious for Valentine. Agramon also appeared to Valentine's prisoner Maia Roberts as her late brother Daniel; she would have died of fear if Valentine had not stopped him.
Agramon later reappeared to Jace in the form of Valentine. Jace guessed that it was not really Valentine and stabbed him. The Fearless rune he'd worn wore off, and for a moment Jace truly feared that he had killed Valentine. Stabbing it destroyed the demon in its corporeal form, however, and it soon dissipated into thin air, sent back to the Void.
When not appearing as someone's greatest fear, Agramon looks big, dark, and formless—like roiling clouds—with big, glowing white eyes. Its eyes remain even when mimicking a human's form.
- City of Ashes (first and only appearance)