Horus was the primarch and master of the XVIth Legion of Space Marines, the Luna Wolves, later renamed by him the Sons of Horus. He was also the first Imperial Warmaster, the most favoured son of the Emperor, and ultimately, the greatest traitor in the history of Mankind.
Horus' homeworld was the Hive World of Cthonia which lay only a few light years from Terra. Horus was thus the first primarch to be rediscovered by the Emperor after the Great Crusade began in the late 30th Millennium.