In Warhammer 30k, it is originally known as the Eye of Terra, the eye represented the Emperor of Mankind's eternal vigilance. Following the Ullanor Crusade and the elevation of Horus Lupercal to Warmaster, the Emperor allowed both Horus and the Sons of Horus to take the symbol for themselves.
Ten thousand years later, the Black Legion chaos marines continues to use the Eye of Horus in its own symbol.