Loyalist Space Marines Chapter of the Second Founding and of unknown lineage, rumored to have close ties to the Mechanicus and the Inquisition. One of their Librarians, Merchsturmer, was one of the two survivors of a Purple Stars ship in a recon mission to the Eye of Terror sent on orders of the High Lords of Terra (the other survivor was the ships Astropath) in the 32nd millenium. Although he was mortally wounded, he managed to extract the Gene Seed of the 5 Marine team sent with him and escaped with the Astropath back to his Chapter and warned the Imperium of another of Abaddon's Black Crusades fast approaching. Presumably, it is the 2nd (597.M32) or 3rd (909.M32) Black Crusade as the 1st Black Crusade happened in 781.M31 (before the recon mission) and the 4th Black Crusade was in 001.M34.
They were created by Barrington Bayley in the 90s, so it's unlikely we'll see them ever again due to the only writer referring to them passing away over a decade ago in 2008.