Linus Geronitan was the 47th Supreme Grand Master of the Grey Knights. Geronitan was chosen as part of a prophecy by the Librarian's of the chapter, who precognated that one would appear who would defeat Mortarion, the Daemon Primarch of the Death Guard.
During the Battle of Kornovin, Geronitan seemingly had his chance to defeat Mortarion. However, the Supreme Grand Master proved no match for the Primarch, who crippled him with a poisonous wind before slitting his throat with his Manreaper. The Grand Masters held a psychic convocation to choose Geronitan's successor and chose Kaldor Draigo. Draigo avenged his master by uttering Mortarion's True Name (the name the Emperor had originally intended for him) and carved Geronitan's name into the Primarch's heart before he was banished back to the Warp.