It was informally referred to as the Autumn, and its name abbreviated to "PoA."
Prior to the Fall of Reach, Pillar of Autumn was selected to support a SPARTAN-II operation to capture Covenant leaders. Captain Jacob Keyes was placed in command. The operation was forestalled by the Covenant assault on Reach and the cruiser was one of the few UNSC survivors and fled the system having lost most of its intended SPARTAN-II passengers.
The cruiser was guided to the Soell system by the AI Cortana. There it was engaged by a Covenant fleet guarding Installation 04. Keyes crash-landed the ship on the installation where the surviving crew continued the battle against the Covenant and later, the Flood. The ship's sole remaining combat effective SPARTAN-II, John-117, was forced to detonate Pillar of Autumn's fusion reactor, destroying the ship and the installation to prevent the installation from firing. [Source: wikia]
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