Purity of Spirit
From Halopedia, the Halo wiki
Purity of Spirit was assigned to Supreme Commander Thel 'Vadamee's Fleet of Particular Justice with her sister ship Undiminished Entelechy. On August 30, 2552, the Fleet of Particular Justice arrived in the Epsilon Eridani system to reinforce Covenant forces during the Fall of Reach. During this battle, Purity of Spirit engaged forces of the United Nations Space Command near Reach. Later in the battle, the ship's crew sent threatening transmissions as a form of psychological warfare to UNSC forces and warships on and around Reach, including to the Halcyon-class light cruiser UNSC Pillar of Autumn.
Towards the end of the Fall of Reach, Pillar of Autumn fled from Reach into slipstream space for the Soell system, where she was pursued by Purity of Spirit and the rest of the Fleet of Particular Justice. Arriving in the Soell system before Pillar of Autumn in September 2552, the Fleet of Particular Justice discovered Installation 04 in the system. Along with Undiminished Entelechy, Purity of Spirit was secretly ordered to conduct a close-range survey of Installation 04 by the Minor Prophet of Stewardship, shortly before the arrival of Pillar of Autumn.