Redeemer

The Redeemer was a monolithic Forerunner dreadnaught, of the Admonisher classification, used as the personal flagship of the last Didact before his final battle and the activation of the Sword Installations and the beginning of the Great Journey. The ship was left in the care of the Archon-class A.I., 03 Ancient Watcher.
At the time of its construction, mere decades before first contact with the Great Enemy, the Redeemer was already outdated. Newer, modular Banisher class Dreadnaughts had become the mainstay of the Forerunner fleet. The Redeemer was slow, underpowered and carried outdated weaponry. However, when the war against the Flood began, it proved that the Redeemer's older computer systems and heavier spaced armour designed proved highly resistant to infection. The Admonisher classes could survive in combat for longer periods of time, since it had much heavier tonnage than both the Banisher-class and Keyship-class, though lacked the high power outputs of either of them.
The Admonisher classification was rushed back into combat status, being hastily refitted to make them combat capable. The Redeemer itself was selected by the Didact following his previous command ship, the Envoy, being crippled in the battle of the Black Verge. The Redeemer was heavily refitted to Didact's own specifications. It was heavily refitted with new Ion and Particle cannon batteries, Warp Torpedo batteries, Interceptor missile batteries, tens of thousands of Sentinels and Sentinel launch bays for numerous combat and repair tasks, MIDAS launch bays and a newly fitted Maelstrom cannon. The Redeemer became one of the most heavily decorated ships of the war, but after Mendicant's breach of the Maginot Line, it was nearly crippled. The Redeemer and it's fleet made it to slipspace just after lighting the Installations and after the deed was done, returned to real space. The ship was abandoned by Didact and the surviving crew, leaving it in the hands of the surviving Archon-class A.I., Ancient Watcher. As Didact went on to the Great Journey, the ship was towed, with thousands of other abandoned Forerunner warships to the Necropolis, where it was integrated into the surface structure. It became the mausoleum where Watcher spent his next few millennia in restful meditation, occasionally waking to observe the Reclaimers and other saved races.
At the close of the Human-Covenant War, when the Covenant tried to excavate the Necropolis and were intercepted by the joint UNSC-Sangheili war fleet, Ancient Watcher roused the ship from it's resting place, along with hundreds of other ships. The huge combined fleet returned to Earth where it obliterated dozens of heavy capital ships itself. Though present at a few other battles of the war, it mostly was used to intimidate the Covenant and at Doisac it never even fired a shot but caused two Carriers and their escorts to surrender.
The ship would slumber again under the sands of Necropolis until the Necros warlord, Pride, attempted to take the Necropolis and it's secrets. Roused to battle once again, the warship accounted for taking out a Jehovah-class Super Dreadnaught with impunity.