Sanguinius was one of the Primarchs created by the Emperor of Mankind, but the Chaos Gods managed to scatter them throughout the galaxy. Sanguinius arrived on Secundus, one of Baal's moon, where he was adopted and raised by a local men tribe known as "People of the pure blood".
When the Emperor reached Secundus, Sanguinius recognized his father and joined his Great Crusade. It is then that Sanguinius created his Legion, the Blood Angels.
During the Horus Heresy, Sanguinius was with the Emperor when Hours reached Terra with his fleet: as a desperate act, the Emperor teleported himself, his son and some Space Marines aboard the Horus cruiser, but they were scattered upon arrival. Sanguinius confronted his brother Horus, now empowered with the full powers of the Chaos, but once he realized there was nothing he could do, he attacked Horus creating a small breach into Horus' armor before succumbing.
When the Emperor fought against Horus, it is believed that Horus could not stand the Emperor's power due to the wound he had received from the attack Sanguinius had done.
Sanguinius on Lexicanum Sanguinius on Warhammer 40,000 Wiki