Onom was a magical staff given by Psymn to Aegyptus Juvens during the latter's travels to found the New City. It had five charges or uses, and would irrevocably destroy anything it hit in anger. Most famously, Aegyptus Juvens uses its last charge to destroy the palace of Ge Diomala in Nelucium, thus forcing the Hypernatural to flee to Exa. Onom was destroyed itself in the process, having expended its charges.
Aegyptus Juvens' use of Onom is cited by many sorcerers as proof he was naught but a rod-boggler.