At the end of the Inter-Legionary wars, the Primarch of the Ultramarines Legion completed his massive work now commonly known as the Codex Astartes. Although this tome described in detail all aspects of the organization of the Space Marine forces, it was mainly renown for the restructuring of the Legions in to one thousand strong chapters. Most loyalist legions followed the Codex and split themselves in to second founding chapters, although not all follow the rgid organizational structures of the Codex. It was also during this time, late in the events of Horus' Heresy that the Dark Angels legion was plit by Luthor's betrayal. Brother fought brother, Luthor sank in to madness and Lion El Jonson, primarch of the Firts legion was never seen again. The survuving loyalists overcame their loss and regrouped, forming a number of chapters according to the Codex Astartes. However, they did vary their organization in a number of ways, the most obvious being the Deathwing and Ravenwing companies and the least visible being the formation of the Inner Circle and their secretive quest for redemption. Similarly the traitorous Dark Angels, found themselves in various situations, being scattered across time as well as space. From individuals acting as mercenaries, to those forming warbands, they spread across the galaxy.
Here within this cavern lie two secretive tomes. The first is The Book of the Unforgiven, and the second is The Script of the Fallen Angels.