An onager being prepped.
An Onager is a small, portable, Roman catapult that was used during sieges.
The Heroes of Olympus[]
The Blood of Olympus[]
Octavian is revealed to have used most of the Legion's treasure to buy Cyclops-built onagers for use in the Siege of Camp Half-Blood. In addition, the Gigantes also have a ring of onagers bought from the Cyclops at the Acropolis of Athens, preventing the Argo II from approaching by air.
In order to defeat the giants, Piper McLean, Percy Jackson and Annabeth Chase sneak into the Acropolis from underground. Although Percy and Annabeth are captured, Piper succeeds in sabotaging three of the onagers, allowing the Argo II to safely approach from the north.
After arriving at Half-Blood Hill, Nico di Angelo, despite his exhaustion, attempts to come up with a plan to stop the onagers so that Camp Half-Blood will be safe and Reyna can bring the Athena Parthenos without risking getting shot down. Nico is joined by Will Solace, Cecil Markowitz and Lou Ellen Blackstone who were sent by the camp to sabotage the weapons. At Nico's suggestion, Cecil sabotages three of the onagers so that their firing lines intersect with the other three while Nico and Lou Ellen protect him. The three are caught by Octavian and his forces as Dakota and Leila arrive with the Fourth and Fifth Cohorts to convey orders from Reyna to stand down. Octavian nevertheless attempts to fire on Camp Half-Blood, but Cecil's sabotage causes the onager payloads to destroy each other. Enraged by the attempted attack, Clarisse La Rue leads an army of Greek demigods out to confront Octavian, but Reyna arrives with the Athena Parthenos, returning it to the Greeks and healing the split between the two sides.
Moments later, Gaea rises on Half-Blood Hill and the Greeks and Romans fight together against her army. In the midst of the battle, Will grabs Nico and takes him to where Octavian is resetting one of his onagers to fire on Gaea where she is battling Piper, Jason Grace, Leo Valdez and Festus in midair, having gone beyond any sort of reason. Unknown to Octavian, his robe gets caught in the onager's firing rope which both Will and Nico notice. Michael Kahale arrives and notices this as well, but he chooses not to inform Octavian and instead to let him fire. Remembering Hades advice that some deaths should not be prevented, Nico prevents Will from interfering. Octavian fires the onager at Gaea, inadvertently firing himself along with the payload. The onager payload joins Leo's supernova of fire in defeating Gaea whose essence is scattered, but Leo is killed in the process.
In the aftermath, Octavian is hailed as a hero who died helping to defeat Gaea. It's suggested that the onager shot may have helped to scatter Gaea, but Nico worries that all it did was kill Leo, unaware that Leo was resurrected by Festus with the Physician's Cure.
The Trials of Apollo[]
The Hidden Oracle[]
Rachel Elizabeth Dare reveals that Octavian had gotten his onagers with the help of Triumvirate Holdings.