Incognito Era

The 425 era was based one year after Halo Wars Two, when the UNSC Spartan's Prayer crashed onto Installation 00, and is named after Jay-425, a Spartan III incognito.

Post-Crash
After the ship crashed, Jay-425, Jayne-428, and Kal-338 managed to locate survivors who weren't already taken out by the Banished. Jayne lead the resettelement effort, while Kal scouted out the enemy. Jay ended up leading a small team of Marines and ODSTs and attacked Banished forces. However, there were too many casualties and Jay was forced to fall back.

Kal found out that the banished had been looking for a composer, and after learning this he reported back to the groups camp, sparking rumors amongst the survivors.

Eventually, the survivors tried to destroy the Banished jamming stations to try and contact the Spirit of Fire, and despite destroying one of the stations, they ultimately failed at that.

Battle for the Temple
Eventually, the survivors discovered a Forerunner temple that held valuable forerunner weaponry, which was how Jay received his boltshot. The banished ended up up invading the temple, and the survivors ended up retaking the temple, with much resistance. Eventually, the survivors took as many forerunner weapons as they could and destroyed the temple.

The Flood
Eight months into the survivor's stay, patrols began to go missing, with no trace of them found. It was originally suspected that this was the Banished's doing, but then those suspicions were dropped when Jay went searching and returned with horrify news; the Flood had returned. The small camp originally was going to look for the composer themselves, that is, until the Banished found it.

Battle for the Composer
After the Banished found the composer, UNSC, Banished, and Flood units clashed, until almost all of the UNSC forces were killed, with the Banished falling back to base. Jay, Jayne, and Kal were the only remaining UNSC forces left.