When the Primarchs were blasted to the outer reaches of space by unknown forces, the Blood Angel's Primarch Sanguinuis had landed on the planet Baal, but he was not alone. Sanguinuis had a twin, a twin who was hidden and even the Emperor did not know about, even though no one knows why. Sanguinuis protected his little brother and taught him all he knew. Acromichus, for that was his name, had a pallid, bloodless look about him, he adored the snow and cold and his wings were not the silvery white of his brother, but instead had a bluish tinge to them. Acromichus was slightly wiser than Sanguinuis, and didn't share his brother's emotion and zeal in fighting. He was, in a way, smarter than his brother, but could never match his strength. Acromichus was revealed to the Emperor in secret, and the Emperor granted them both space marines. But enraged Acromichus as he gave to him merely three thousand marines. Acromichus saw this as his brother's fault, and he started a bitter feud against the Blood Angels. However he did not, for even one second, doubt the Emperor's judgement or was ever angry or mad at Him. He kept his dignity in front of the Emperor, swore his allegiance to Him and accepted the meagre amount of space marines with great reverance, naming them the Frost Angels, masters of ambush and strategic warfare. From that day on every two years, a squad of especially trained marines was pitted against another in an enclosed arena, the two primarchs led the squads and the winners were granted great honour. After Sanguinuis died however, the rivalry had vanished almost instantly. Every Chapter Master was chosen by the current one years before he died, and each new chapter master went through a revered though extremely painful process of receiving bio-mechanical wings which were inserted into their backs without any sedatives or painkillers. If the subject died, then the runner up would get his wings inserted, this would allow only the strongest and wisest to get power over the chapter. After Roboute Guilliman wrote the Codex Astartes, he allowed the Frost Angels to keep all their space marines and not split up their chapter to honour Acromichus who died defending Terra's people with two companies of Frost Angels from the murderous rampage of the Emperor's Children while the rest of the chapter protected the Emperor's Fortress. It is said the Acromichus went into a blustering rage once he recieved the transmission that his brother had died, and kept fighting long after his companies had died, yelling oaths of forgiveness for his brother and hatred for chaos. After the Heresy, the Frost Angels and Blood Angels conducted many crusades together, the Frost Angels setting up ambushes, spying and otherwise helping the Blood Angels behind the scenes to help get rid of Chaos. They also assisted Blood Angels under the Black Rage and there are some reports of Frost Angels with the Black Rage in their gene-seeds. Inquisitor Sinus Kazreston once said about the two chapters, "So different yet so much alike".