From Halopedia, the Halo wiki
The UNSC Eternity is an Infinity-class supercarrier manufactured by the Special Assembly Plant Concord. It is intended to be the sister ship of the UNSC Infinity, flagship of the UNSC Navy. By late September 2558, the Eternity was still incomplete. At this time, pre-manufactured modules of hull sections and weapon systems intended for the Eternity were instead redirected for a maintenence and overhaul cycle the Infinity underwent over Earth following damage it sustained during the recent Battle of Aktis IV and a prior ambush by the New Colonial Alliance and an ex-Covenant mercenary group.