Siege of Zintheron II

The Siege of Zinthreaon II was a massive joint-Imperial defensive campaign conducted by the Golden Knights and later supported by elements from the Ultramarines, Space Wolves, Blood Angels and several of their Success Chapters, to halt a massive incursion by the Traitor Legions against the world of Zintheron II.

Background
The earliest, officially recorded conflict in the Golden Knights Chapter annals that bears full information. The siege of Zintheron II began when large elements of all nine Traitor Legions attacked the world of Zintheron II, when a warp storm opened up next to the system's sun. The assault was orchestrated by the Daemon Primarch Perturabo and the Warmaster of Chaos, Abaddon the Despoiler.

In the early days of the siege, the Chapter's fleet, as well as the Sector Defense fleet, engaged in a brutal stalemate against the traitor fleets. During the course of this space conflict, elements of the Night Lords and Alpha Legion Traitor Legions made planetfall and engaged the recruit companies, dealing heavy casualties to the recruits in the process before fleeing to the underbelly of the hive city as the bulk of the chapter arrived.

Utilizing the diversion, an Alpha Legion infiltration team, led by a warrior that referred to himself as Omegon, though it remains unknown if it was the Primarch Omegon or a common astartes claiming to be Omegon, entered the planetary Fortress-Monastery of the Golden Knights and stole numerous records, before destroying what he could not take, and fled along with the entirety of the Alpha Legion forces that took part in the conflict. After weeks of orbital conflict, the defending fleets were forced to flee from the system. Soon after, the remaining forces of the Traitor Legions made planetfall at the south pole of the planet, far from the Golden Knights' fortress.

The bulk of the traitors forces began assaulting the lines of both the Golden Knights and their Imperial Guard allies. Meanwhile, elements of the Iron Warriors Traitor Legion deployed behind the Golden Knights' lines to destroy their frontline supply posts, and at the same time, the Night Lords, whom came out of hiding after the other traitors made planet fall, would attack key manufactorums in the heart of the hive before elements of the Custodes Regent could respond.

By 174.M38, the Traitors had pushed the forces of the Golden Knights back to their fortress-monastery, taking extreme casualties in the process. After over a year of fighting, the Golden Knights were on their last legs, with only about three hundred Astartes remaining. However, the siege was lifted when the Golden Knights fleet finally returned, along with a detachment of Ultramarines, Space Wolves, and Blood Angels, as well as several of their Successor Chapters.