"Halcyon-class ships (…) have a reputation for being virtually indestructible."
The UNSC Pillar of Autumn is an iconic ship in the Halo Universe and Franchise. It gave the very first impression of Halo, as it was the very first object seen in Halo: Combat Evolved. When it was first seen in Halo: Combat Evolved, it was already over 40 years old and now captained by Captain Jacob Keyes.
The Pillar of Autumn, informally known as the Autumn, is a Halcyon-class light cruiser designed by Dr. Robert McLees in 2510. The design of these Halcyon-class ships are infamous for their unbreakable hull. Ships of the class were able to remain operational despite sustaining breaches to all compartments and losing 90% of their armour. This resistance to battle damage is due to its unique internal design, the ship is built with a series of internal cross bracings and honeycombs. This design led to the very short-lived service life of the Halcyon-class ships, as this made the Halcyon-class ships expensive to build. This led to a radical design change, as all 50 commissioned Halcyon-class ships would have received this internal structure but only the first 11 received their honeycombed structure.
The high maintenance cost led the Navy branch of the UNSC to commission replacement ships. This new class of ships would be known as the Marathon-class, and would replace all the Halcyon-class ships. The decommissioned Halcyon-class ships were put in 'storage' on the planet Reach to be revived if/when needed. It wouldn't be until the Human-Covenant war erupted in 2525 a number of Halcyon-class ships were reactivated from storage and refitted.
"There was only one ship."
At Reach, Keyes was given an important mission by Rear Admiral Stanforth: to transport SPARTAN super soldiers deep into Covenant space in order to capture a Prophet. The Prophet would then be used as ransom to end the Human-Covenant war. This mission was aborted shortly after preparations were finished, as the Covenant attacked Reach. He offloaded most of the SPARTANs on the planet Reach, and sent three to Reach Station Gamma. The Pillar of Autumn then fought in the ensuing battle, and even managed to destroy the Supercruiser flagship. The Autumn was just able to pick up John-117, Linda-058 and a handful of Marines before being forced to flee the fight. The Autumn was just able to make a quick stop in the Aszod ship breaking yards to retrieve a fragment of Cortana from Noble Six. After the package was retrieved, the Autumn quickly fled into Slipspace, now controlled by Cortana.
The Autumn was now in uncharted territories, shuttled away towards a, albeit not totally*, random trajectory. This trajectory led to the first discovered Halo Array Installation 04, sometimes referred to as Alpha Halo. By this point, the Autumn was damaged during the Battle of Reach and was in awe at the ring structure before them. This resulted in a Covenant ship boarding the Autumn, using the empty places from the Autumn's escape pods to attach its pods. Captain Jacob Keyes then activated Cole Protocol Article 2, which meant that Cortana would be given to John-117 to prevent her being captured by the Covenant Empire.
As the Autumn was now without its AI, Captain Jacob Keyes took manual control. While still being attacked by several Covenant ships, the Autumn was put into a dive towards the Halo Array. Eventually the Autumn did crash on Alpha Halo, damaging it enough that it would not be able to leave the ring unassisted. However, the Autumn was still relatively intact, mostly due to its infamous honeycombed structure. When John-117 found out that the ring they had crash-landed upon was in fact a weapon on a galactic scale, he decided to use the Autumn's engines to destroy the ring.
John-117 fought his way through the ship to reach Engineering. Following Cortana's instructions, he destroyed each of the four engines by throwing grenades and firing rockets into the open shafts, destabilizing the containment fields. Cortana predicted that they had 15 minutes before the engines detonated. John headed for the elevator where he was ambushed by Zuka 'Zamamee. With Cortana's help, John killed Zuka by knocking the Sangheili to the bottom of the shaft. John then used a Warthog to drive along the length of the Autumn to a location where John was to be picked up. However, two Banshees shot down the transport. John was then forced to drive another kilometre to where a single Longsword fighter was docked. The Spartan successfully boarded the ship and escaped Halo. Moments later, the Autumn's engines detonated, destroying Halo and bringing the battle to an end.
* The trajectory Cortana plotted was based on some Forerunner symbols she discovered. It was not exactly in line with the Cole Protocol, but it did not lead back to Earth.