Horus Lupercal, otherwise known as FUCKING HORUS by the Emperor, was the former Warmaster of the Great Crusade, Leader of the Traitor forces, Primarch of the Sons of Horus (now the Black Legion), and sixteenth son of the Emperor. He was responsible for leading the great civil war known as the Horus Heresy, in which he and half of all Imperial forces were corrupted by Chaos, and attempted to destroy the Imperium of Man.
Official Canon
Horus was the first of the Primarchs to be discovered by the Emperor on the planet Cthonia; thus, out of all his brothers, he has the most combat experience and direct connection to the Emperor. His legion, originally known as the Luna Wolves, were an excellent workhorse legion that participated in every major offensive of the Crusade. Horus himself was also a great leader, both on and off the field, as he was able befriend every single one of his brothers.
After the victory of the Ullanor Crusade, it came as no surprise that Horus received the highest honors among his brothers for crushing the largest Ork Empire in the galaxy. However, much to the dismay of the primarchs, but especially Horus, the Emperor announced that he was returning to Terra for reasons he could not tell his sons, and put Horus in charge of the Great Crusade.
Horus Heresy
Following this, doubt crept into his mind, and thus the already corrupted Lorgar and his co-conspirators Kor Phaeron and Erebus set to bring about Horus' downfall. They engineered the destruction of the Interex, who could have warned the Imperium of the existence of Chaos. Meanwhile, Horus was mortally wounded and brought to a warrior lodge where he received visions from the Chaos Gods of a distant future, where the Imperium was crumbling and the Emperor was worshiped as a God. Enraged, he renounced his loyalty to his father, though unwittingly, the future he saw was one he helped to make.
Horus used his contacts to sway the other primarchs to his cause and secure war assets, until finally he struck on Istvaan III, by destroying the remaining loyalists in the Traitors' legions. The loyalists were caught off guard, as many other legions were suddenly betrayed by their former allies.
This long and bloody civil war finally culminated in the Siege of Terra, where Horus personally lead the traitor forces. In the end, he challenged the loyalists to come for him on his own flagship; Sanguinius reached him first, and while he put up a strong fight, Horus slew him and destroyed his soul. The Emperor reached Horus too late, and saw the broken body of his son. As they dueled, the Emperor struggled against his own son as he had been blessed by the Chaos Gods themselves. It was only after the intervention of Ollanius Pius, a single guardsman, that the Emperor was able to use his pyschic might to destroy Horus's soul, thus ending the Horus Heresy. However, in the end Horus succeeded in mortally wounding his father, thus did he get the last laugh.
TTS Canon
Because Horus' soul has been completely destroyed, the one place where he can continue to exist is in the Retconnian, along with Malal and the Squats. Horus appears to have given up on his ambitions and consoles a bitter Malal with promises of Hot Chocolate.