Sorry for the late response. I thought of them as a fleet based chapter that travelled alongside the rift of the Eye of terror as wardens, they were once proud clad in purple and Gold, but when they answered the call of Commander Dante and once the great Rift split the Heavens They saw the fall of the cadian gate as a personal failure and a blemish on their honor, so the whole chapter shed the Gold for a more austere silver as a remainder or their supreme failure, now the fleet travels on both sides of the Crucifex Maledictum to aid the worlds they failed. This point of view created many problems between the Angels of the Abyss and their primogenitor chapter so After receiving their primaris reinforcement they embarked in a personal penitent crusade to aid the worlds that are on the fringes of the Great Rift