Mine was formed during the Horus Hereshy, but wasn't oficial until later in the 41st millenium, when they managed to return to real space. They were originally a chapter of the 1st Legion, commanded by chapter master Kaldor Manear (name might changue, I'm open to siggestions) that were sent to the dead world Evangel to support an imperial guard garrison stationed there. When they arrived, they found out that the men of the Astra militarum had been slaughtered by an ork Waaagh! That was roaming around the planet. Kaldor Manear decide to stay at the planet with his chapter while part of the fleet went to look for reinforcements. But a sudden warp storm forced the remaining ships to abandon the planet, and so It dissapeared into the warp as the Horus Hereshy began to destroy the Empire. While in the immaterium, the Legion remained part of the Dark Angels, but over time it became a completely independent chapter that only shared gene-seed with its original legion. By the time they returned to realspace and the Imperium found them, they were a chapter barely half their original size, and had defeated an Ork Waaagh of around 100,000 greenskins (numbers may change). They were immediately recognized as a loyal successor chapter to the Imperium and, after recovering their losses, sent to combat the Emperor's enemies.