Agramon, a Greater Demon, is the Demon of Fear. Agramon takes the shape of its target's greatest fear, possibly scaring the being to death. It lacks its own true physical form, other than its malformed, enormous, hideous shape that seems like a dark cloud of gas with big glowing eyes the size of saucers.
Agramon was summoned by a young warlock named Elias for Valentine Morgenstern in 2007. Upon being summoned, Agramon managed to break free from the pentagram because Elias' greatest fear was of any demon escaping his pentagram. Agramon proceeded to kill Elias, but was put under Valentine's control through the Mortal Cup.
Agramon and Valentine went to the Silent City to retrieve the Mortal Sword. All the Silent Brothers present feared Agramon and died the same evening. Jace, who was being kept in the underground cells, grew scared of the sound the Silent Brothers made and the aura Agramon gave off. Agramon later showed himself to Jace and merely knocked him unconscious.
Aboard Valentine's ship, Agramon showed Jace his greatest fear: a gravely injured Clary. Seemingly dying in his arms, Agramon again knocked Jace unconscious for Valentine. Agramon later appeared to Maia Roberts as her late brother Daniel; she would have died of fear if Valentine had not stopped Agramon.
Agramon later reappeared to Jace, who was wearing the Fearless rune, as Valentine. While Jace had guessed that it was not really Valentine, he was overcome with shock when he stabbed Valentine and appeared to have truly died, before the body dissipated into sand and then into thin air.