They were created as a fail-safe by the Blood Angels, in case the Imperium ever turned upon the Sons of Sanguinius. One reason being their blood drinking rituals and Gene-seed curses could have them labeled as heretical deviants, by the Inquisition or Ecclesiarchy. Should that occur, the Angels Erythrean would be alive to continue their Primarch Sanguinius' bloodline and heritage. However in order to ensure that occurred, the newly created Chapter was sent into the Ghoul Stars and ordered to not make contact with any Imperial forces. As time went on afterwards, future generations of the Sons of Sanguinius forgot all about the Angels Erythrean's existence. The Fleet Based Chapter, though, not only remembers their purpose, but also fights to prevent any threats within the unexplored eastern galaxy from reaching the Imperium. Their task is made difficult, however, due to the Angels Erythrean's self-exile, which prevents them from easily obtaining the resources other Chapters have access to.
Because of this, the Erythrean rely on copied or surviving fragments of technology to continue functioning, which all resides on their fleet's flagship. The Angels Erythrean have also resorted to inducting sub-Human Aspirants into their Chapter and use their technology to turn them into fully functioning Space Marines. The Erythrean's Priests meanwhile, are always faced with the dilemma of whether the process has successfully freed these new Angels of any possible taint they may have possessed. While the Chapter can do nothing about that problem, the Angels Erythrean have long resorted to raiding and reaving, in order to gather resources. This includes targeting their fellow Imperials and ships have gone missing, after being attacked by the Erythrean. They also have managed to make trading deals with their fellow exiles the Carcharodons, as well as other nomad predation forces that dwell in the eastern darkness beyond the Imperium's realm. It is through these various means, that the Angels Erythrean obtained the material needed for them to be able to create Primaris Space Marines on their own.
- 1: Warhammer Youtube page: The Warhammer Community Podcast: Episode 28 – Sons of Sanguinius 37:38 (Posted on 13/01/2021) (Last accessed on 2 February 2021)