The Astartes Praeses were founded as a group of Space Marine Chapters tasked with guarding the borders of the Eye of Terror. It is a difficult and unforgiving task; the chapters are constantly beset by the Traitor Legions and even the occasional Black Crusade. Originally numbering twenty Chapters, eighteen now remain because one went traitor and the other was destroyed. And then this happened.
The full membership of the Praeses is unknown, but the following 14 Chapters have been confirmed as being part of the Praeses:
- Angels Eradicant
- Black Consuls until they were (probably) destroyed. Confirmed to have been destroyed in a suicide nuke in a hive city to wipe out Iron Warrior attackers and rebuilt with Primaris Marines.
- White Consuls, their counterpart, (definitely) still around
- Brothers Penitent
- Crimson Scythes
- Iron Talons
- Knights Unyielding
- Marines Exemplar
- Night Watch
- Viper Legion
- Fire Claws, until they went rogue (kinda, it's in the radical inquisitors sense) and renamed themselves the Relictors.
The Ultramarines Honour Company is sometimes numbered amongst them. It is not an actual Chapter, but is tasked with the same purpose as the Astartes Praeses.
List of chapters that may or may not also be members of the Praeses:
- Sons of Malice - Participated in the 13th Black Crusade to reclaim their homeworld Scelus, a world in the Cadian sector. Which puts them existing exactly where you would want an Astartes chapter if you're trying to round up enough to guard the Eye of Terror. Shame about that whole being excommunicated and not-worshipping Malal thing.
Taking all of this into account, and assuming that the Black Consuls were the aforementioned destroyed chapter and the Sons of Malice were the traitor chapter, and further assuming the Ultramarines Honour Company is not technically counted, that leaves 5 chapters that haven't been named.