The Pillar of Autumn was a Halcyon class cruiser that was in service with the UNSC and it is where the first level of Halo takes place. While the Halcyon class of cruisers were described as old and lacking mobility and firepower they had one redeeming quality; they were extremely tough. They were able to take tremendous damage before their main components would fail and this was in part due to their honeycombed Hull. The Pillar of Autumn was originally supposed to be decommissioned and then scrapped as the newer Marathon class Cruisers were seen as more attractive options. However, the Autumn was reassigned and was chosen for a highly dangerous and top secret mission that required a daring captain and a ship that was capable of withstanding tremendous damage.
Ever since the war with the Covenant started, humanity had been suffering tremendous loses and ONI and the top brass of the admiralty had been seeking a way to end the war. It was decided that the SPARTAN-II's would embark on a very risky mission to capture a Covenant vessel and take it to the Covenant home world where they would capture a Prophet, one of the leaders of the Covenant. It was the hope of the admiralty that such a move would enable them to dictate a peace settlement with the Covenant, who until then had shown no signs of stopping. They needed a daring captain to pilot a ship and to disable, not destroy, a Covenant ship thereby facilitating the SPARTAN's mission. To this purpose, Captain Jacob Keyes was assigned and assisting him was Cortana, who was specially created by Dr. Catherine Hasley, the founder of the Spartan project for this mission. Also assisting them was a large contingent of ODST troops under the command of Major Silva. This mission was believed to be so important, that Keyes was also assigned a Second A.I; Wellsley who would be in charge of point defense. However, the subsequent attack on Reach before their mission began forced Keyes to deploy with what personal he had and thus the second A.I. is never seen in game, although he makes an appearance in the Halo novel.
In preparation for their mission the Autumn was given a major refit, including new engines that were capable to being pushed up to 300%, a new MAC gun that could fire up to 3 shots in one charge and 7800 Archer missiles. Also, the already strong hull was further reinforced by a number of bulkheads and the offensive armament of the Autumn was increased with the addition of 3 Shiva nuclear warheads that were capable of being put on Longsword interceptors and flown remotely. All this armament made the PoA arguably the most powerful ship in the UNSC fleet.
Although the PoA was about 43 years old upon its destruction, it functioned flawlessly till the end. In spite of the fact that the covenant forces had overwhelming superiority, both in numbers and technology at Reach and then at Halo, the PoA stood her ground and was able to take a tremendous beating. Exact details are not available but is known that the PoA destroyed/disabled about 5 CCS-Class Battlecruisers at Alpha Halo in addition to the assault carrier, a frigate, numerous drop ships, scores of fighters and a Supercruiser at Reach. More importantly, the ships experimental engines were able to allow the autumn the unique advantage of exceeding terminal speeds in tough situations. It was able to race to help defend Reach, while a conventional ship would have taken a much longer time. The reinforced bulkheads and the secret mega arsenal along with the triple shot MAC cannon were able to dish out huge damage without succumbing to critical damage. Thanks to Keyes and the chief, the autumn was able to carry out another vital mission; the destruction of the navigation computer aboard a ONI prowler that had failed to wipe its navigational data clean. Had this data been activated by the covenant, it would have shown anyone who accessed it a clear map to Earth and the location of the inner planets. After its escape to
Halo, the PoA fought on and it destroyed 4 CCS class cruisers and is known to damaged one other cruiser, the truth and reconciliation. It should be noted that the High prophets had forbidden the Covenant warships above halo to use their plasma torpedoes over fear of damaging the "sacred rings'. Regardless, the ship stood its ground until Jacob keyes initiated cole protocol article 2 and ordered the crew to abandon the autumn while he tried to "land' the ship on halo. He crash landed the ship on the surface of the ring and the ship came to rest on a high valley; Cortana later remarks that the ship is still functioning but will never fly again. In an effort to stop 343 Guilty Spark from activating the ring, the chief along with cortana detonate the ships reactor and destroy the ring.
All in all it can be argued that the PoA was probably the most successful ship in the covenant-Humanity war as it was able to destroy 6 Capital ships, including one AC and one Supercruiser; which were each over 5 times as massive as the autumn. In addition to that it destroyed dozens of fighters, drop pods and a heavily armed covenant frigate in addition to severely damaging another capital ship( CCS class cruiser, the truth and reconcilliation) in just 2 brief battles.