From Halopedia, the Halo wiki
The Io was heavily damaged during an engagement in the 18 Scorpii System, where it and two other ships were saved by the timely arrival of the frigate Bellerophon and other Insurrectionist ships. Afterward, Admiral Preston Cole deemed that the cruiser was damaged beyond repair and intended to follow the recommendation of a Captain Wren to use it as a "fire ship".
The ship was at the Battle of Psi Serpentis and was destroyed. Later speculation indicated that Admiral Cole may have used the ship as a decoy to convince both the Covenant and the UNSC that he was killed during the action when he may have in fact escaped in his flagship, the UNSC Everest.
It is most likely named after the moon of the same name.
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