Rho-Mu 31 was a Skitarii Protector during the Great Crusade and Horus Heresy. Actually consisting of an entire squad, Rho-mu 31 was a collective and shared identity. Serving under Tech-Priest Koriel Zeth, Rho-mu 31 was assigned to rescue and guard the Psyker transcriber Dalia Cythera shortly before the Schism of Mars. During the Schism, he took part in the battle against the Kaban Machine alongside Dalia. When Dalia was the new Guardian of the Dragon, Rho-mu pledged to stay by her side and protect her inside the Noctis Labyrinth. Unusually for Skitarii, Rho-mu 31 was capable of some forms of humor and emotion.