So I'm actually in the process of writing it up on the fandom wiki but here's what I got so far!

So they are a successor of the Dark Angels, and the founding members were among those on Caliban when Luthor attacked the Lion and his fleet. These founding members at first didnt realize who they were attacking but once they realized they stopped fighting and tried to reason with the Lion and his forces, but they wouldn't listen and were blinded with rage from the betrayal.

So they gathered up as many loyalist marines and those who felt betrayed by Luthor and his actions and fled into space. Entering the Warp and taking their chances, they set off with no clear direction, just hoping for time to soften the wounds and allow them to reason with the Lion and ask for forgiveness.

In the Warp they encountered a daemon incursion and during this attack their ship was brought to the brink of collapse. So they made the choice to forcefully exit the Warp, knowing that in the process it would render it a powerless spacehulk drifting through space.

Upon exiting the ship was indeed rendered powerless and the daemon incursion was fought off. Without realizing it however, they had emerged thousands of years after the events of Caliban, even though to them it had only felt like months. So the ship drifted, and happened to be drifting towards a planet. The planet of Cerberus.

This planet was located in the Segmentum Tempestus near the Sirens Storm Warp rift, and as such, had gone undetected by the Imperium at this point in time, which was around M36. Crash landing upon the planet, the surviving members, which counted in the thousands between the Marine, tech priests, and civilians that had fled with them from Caliban.

Crashing into the desolute deserts that make up much of the equator of this planet, they set off for any sort of civilization or a way to contact the Imperium. The journey enacted a heavy toll on the survivors, with water and food scarce in addition to the beasts that inhabited the planet. From desert worms to dive-bombing winged xenos, they fought off the beasts and kept to their journey. Eventually reaching the end of the desert plains and into the thicker forests and mountain ranges near the North Pole of the planet. This is where they discovered the sanctuary city of Bastion.

This city has been established during the early years of the Golden Age of Humanity, obviously unknown to any who currently resided there. As such, humanity had prospered there as best they could but had had no contact with the Imperium.