The Betrayal

'The Betrayal is the name of the lamentable event that saw the downfall of the once-proud Sworn of Guilliman Space Marine Chapter, which led to their being deemed as renegades and later outcast from the Imperium. Following their excommunication, the newly renamed Chaos Warband - now known as The Betrayed - these renegades continue to slowly drift towards pure chaotic corruption, even if its members are still far from true believers, instead favouring the use of minor daemons as auxiliaries in their raids.

Background
In 949.M41, a call of distress came from the minor Forge-World of Haphastus VII, calling for the full might of an Astartes Chapter to come and protect it from Heretek and Chaos Marines assaulting it. The Sworn of Guilliman Chapter answered with speed, gathering its full might and delved in the Warp. After six months, they finally reached Haphastus VII only to discover a perfectly functional world which had never called for their help. Instead of being held as saviours, the Sworn were deemed suspicious, coming uncalled and in full-force, their weapons ready to fire on the Forge-World. After months of debates, the Sworn finally returned to Santiro having cleared their name and were preparing to discover who had sent the false call of distress.

It was not to happen for, when they exited the Warp near Santiro, they were assaulted by forces of two Adepta Sororitas orders, several warship of the Imperial Navy and even forces from Hephastus VII, all led by High Inquisitor Calgae. While their starships were destroyed one after another, the Sworn of Guilliman learnt that they had been declared Excommunicate Traitoris for the attack on Hephastus and the murder of an Inquisitorial task force sent on their homeworld. The Sworn tried to fight back but they were outnumbered and outgunned vastly and their only chance of survival was to flee their homeworld system. Yet, in a daring action, the forces of the Chapter still holding their Fortress-Monastery managed to send their archives and gene-seed stocks in space by using Thunderhawks, which were brought into the Might of Santiro, the least deteriorated spaceship of the Chapter. Losing more forces in their manoeuvres, the Sworn managed to escape to the Warp only barely.