The Nameless One
The Nameless One, the Lord of Chaos, the Father of the Gods, Chaos Ascendant, is a primordial Warp Entity that is in fact not a Chaos God. While the Gods are born from mortal emotion and are single minded in nature, The Nameless One's existence predates the mortal races, and perhaps even the Materium, as it has existed as long as the Warp itself has existed. Originally, it was the Warp itself, and omnipresent entity in the Warp. But after feeding off the emotions of newborn races, it unwittingly birthed the 4 Chaos Gods. They overthrew their predecessor, shattering it into billions of shards, including Furies and Enslavers. Since then, it has plotted it's return. Any action in the name of Chaos Undivided gives The Nameless One power. He has claimed the Black legion as his own, and marked Abaddon with the Mark of Chaos Ascendant, as a sign of the Despoiler's unknowing part in his scheme. Since it does not need the mortal races to survive, it cares not for eternal war, and instead seeks a final victory for Chaos. Essentially, he is Chaos in all it's forms. The Nameless One is worshipped by a Chaos Warband called The Daemonkin, a host of Word Bearers who act as much like the other Word Bearers as possible, but secretly harbor their discovery of The Nameless One.
The Nameless One is also the source of Komus, the Tyrant Star, and the Star itself is the harbinger of The Nameless One. The darkness the Tyrant Star's prophecy refers to is actually the Nameless One, and if not stopped, it will not only devour the realms of Man, but all that exists.
The Nameless One is the source of all that is Chaos, and without it, the Warp would not exist. Much of it's nature is incomprehensible, but it is known that all the Chaos Gods, both major and minor, tremble at the thought of this entity.
The Nameless One's goal is to devour all other Gods, both Chaotic and otherwise, and rule over both the Materium and the Warp. It has already succeeded in one universe, due to The End Times of Warhammer Fantasy Battle. It's Everchosen, Archaon, has succeeded and plunged the world into Chaos. The human, dwarven, and elven gods are devoured by the Nameless One, as the other Gods do not simply leave in boredom, but in fact flee for their lives. It's main interest, however, is in the Warhammer 40,000 universe, because of it's sheer size and number of mortal life forms. If it's chosen champion, Abaddon, slays the Emperor of Mankind, then the Nameless One will devour all Chaos Gods and minor deities, and regain it's original place as the ruler of the Warp.
In both the Fantasy and 40k versions, The Nameless One is the driving force behind all forms of Chaos Undivided, including the Everchosens, and the Daemon Prince Be'Lakor.
It is also responsible for the nonexistence and destruction of Malal. You do not want to know what happened to him. (The Nameless One ate him)