Undiminished Entelechy

The Undiminished Entelechy was a CCS-class battlecruiser which was part of the Fleet of Particular Justice, along with its sister ship, the Purity of Spirit. It took part in the Battle of Installation 04, but was destroyed in the battle.

Fleet of Particular Justice
The Undiminished Entelechy and Purity of Spirit were two of the very first ships to be assigned to the Fleet of Particular Justice, under the command of Thel 'Vadamee. The two ships remained in the fleet throughout their operational history.

Battle of Installation 04
During the Battle of Installation 04, the Undiminished Entelechy and the Purity of Spirit were ordered to scout the surface of Alpha Halo in advance. In doing, the Undiminished Entelechy picked up many valuable Forerunner artifacts.

Later in the battle, the escaped Flood managed to board the vessel, and Thel 'Vadamee subsequently ordered the vessel to be destroyed. It crashed upon the ring shortly before the ring was destroyed. It, and its treasure trove of Forerunner artifacts, passed out of memory and records of both the UNSC and Covenant, eclipsed by Halo's destruction. The ship's wreckage was swallowed by plant growth which was stimulated by fail-safe mechanisms in the ring.

However, at a later date, the Office of Naval Intelligence acquired scans of the ship's geometry and integrated them into the War Games simulations, for unknown reasons.

Trivia

 * In some UNSC simulations, the ship's class had been changed from a CCS-class battlecruiser to an unidentified one, in order to train tacticians.