|“||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 , Elias, for Valentine Morgenstern in September 2007. Upon being summoned, Agramon managed to break free from the pentagram he was trapped in. This was because Elias's greatest fear was any demon he summoned breaking out of its pentagram. Agramon then proceeded to kill the young warlock, after which Valentine began controlling him via the Mortal Cup.
The duo later went to the Silent City to steal the Mortal Sword. The Silent Brothers present were not immune to his powers, resulting in them being scared to the point of death. Jace, who, at the time, was incarcerated in the underground holding cells of the Silent City, was alarmed by the mysterious sounds above ground. The aura given off by Agramon's presence was a powerful and terrifying one. Agramon, revealing himself to Jace, knocked out the latter.
Aboard Valentine's ship, Agramon, using his powers, showed Jace his greatest fear: a gravely injured Clary, seemingly dying in his arms. The demon proceeded to render Jace unconscious afresh. Agramon also appeared to Valentine's prisoner Maia Roberts as her late brother Daniel, almost killing her. She would have died had it not been for Valentine.
Agramon later reappeared to Jace in the form of Valentine. Jace ventured that it was an impersonator, and stabbed him. The Fearless rune he'd worn wore off, and for a moment Jace truly feared that he had killed Valentine. The stab destroyed the demon's 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)