From Halopedia, the Halo wiki
He was aboard his ship during the Battle of Installation 05 and when the Great Schism broke out, he took control of several other Jiralhanae-held ships in the Second Fleet of Homogeneous Clarity and brought them to attack the Sangheili ships. His ship, along with the Tenebrous, tried to attack the Sangheili ship, Incorruptible, but the attack was stopped when the Tenebrous was destroyed by the Incorruptible and the Twilight Compunction was attacked and damaged by the Sangheili carrier Lawgiver.
Gargantum's ship, along with the Revenant and another frigate, followed the Incorruptible as it tried to escape. When the ship Bloodied Spirit dropped out of slipspace nearby, Gargantum hailed it and discovered it was controlled by a team of Spartans.
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