Official Canon Edit
Sanguinius is unique among his brothers in that he possess a pair of massive, angelic wings, giving him the capability of true flight. While such a feature would normally condemn one as a mutant, Sanguinius won over the people of Baal as he united its people against the mutant marauders living in the desert. When the Emperor arrived and sought out his son, Sanguinius immediately recognized him as his true father, and pledged his loyalty to him on the spot. When reunited with his legion, the Blood Angels, Sanguinius taught his sons humility and to seek beauty in war. They are among some of the most renowned craftsmen in the Imperium. They also put the welfare of the Imperium and its people over seeking personal glory in battle.
Horus Heresy Edit
At the dawn of the Horus Heresy, Sanguinius had his first brush with Chaos when he encountered the Bloodthirster named Ka'Bandha. As the two dueled, Ka'Bandha cursed the Blood Angels, before running off. Now stranded, the Blood Angels eventually made their way to Macragge, where Roboute Guilliman and Lion El'Jonson named Sanguinius the Regent of the Imperium, until proof emerged that the Emperor was alive.
Unwilling to wait for the enemy to make their first move, Sanguinius braved the Ruinstorm ahead of his brothers and arrived on Terra, where he, Jaghatai Khan, and Rogal Dorn were alone against the combined hordes of Chaos. Sanguinius dueled Ka'Bandha again, this time cracking the Daemon's spine and hurling him into the warp. In the final duel of that battle, Sanguinius fought first against Horus. But as Horus was now blessed by the Chaos Gods, he defeated Sanguinius and shattered his soul, just in time for the Emperor to see his ruined body.
Upon his death, Ka'Bandha's curse took effect, and the Blood Angels now faced a horrific madness known as the Black Rage. Sooner or later, any battle brother will become unyieldingly furious as he has vivid hallucinations of Sanguinius' duel with Horus. However, the Blood Angels can also be saved by a mysterious figure known as the Sanguinor, a being sharing Sanguinius' likeness that appears from the warp in their hour of greatest need. Some speculate that he is the last remnants of the Primarch's better nature, but what he truly is is unknown.
Sanguinius' sacrifice is remembered to this day in the holiday of Sanguinala.
TTS Canon Edit
The Emperor is still deeply saddened by Sanguinius death, sobbing (or the closest approximation his Text-To-Speech device will allow) whenever he thinks of him. And unlike his brothers, the Emperor allows no one to insult his "Fabulous Fucking Hawk-boy" due to sacrificing himself. When asked if the Sanguinor was Sanguinius, the Emperor simply said that if he was even a shard of his son, he wanted him to know that Daddy missed him very much.