Agramon, a Greater Demon, is also known as 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 he summoned on a job escaping its pentagram. Agramon then proceeded to kill Elias, before being 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, an all of the Brothers in the Silent City at the time of Agramon and Valentine's attack were ultimately scared to the point of death. Jace, who was being kept in the underground cells, grew scared of the sound the Silent Brothers made as a result of their fright in addition the aura given off by Agramon's presence. Agramon later revealed himself to Jace, though he only knocked him unconscious, likely under Valentine's orders.
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, in the form of Valentine. While Jace had guessed that it was not really Valentine, he was overcome with shock. Jace then stabbed Valentine, who appeared to have truly died, before the body dissipated into sand and then into thin air.