"The Emperor shines to us as a beacon in the Warp! All who gaze upon it must fall in line or fall to ash! Yet, we care not which you choose."
― Arcas the Infernal, Chapter Master of the Soul Burners, speaking to the civilian population of planet 132-19.
The Soul Burners, are a loyalist Space Marine Chapter of founded sometime shortly after the Age of Apostasy. Their parent chapter is unknown to the Imperium at large, kept a secret by the Inquisition and the Chapter for unknown reasons. The Ordo Hereticus keeps a close eye on the Chapter, and it's Librarius specifically, sending the Chapter in alongside the Adepta Sororita as a sort of second Chamber Militant for the Ordo. As such, their numbers are larger than an average chapter.