The Burning Eye
Cyphiax is a demon lord who’s planar holding in the Abyss came in to contact with the plane now called Armageddon. He has since launched an invasion of that world.
Among his most powerful agents are the Four Horsemen.
He is known to reside on Apocalypse, a plane he has personally spent decades destroying and bringing ever close to the Abyss. After joining forces with the mysterious Cult of the Shadowlord, he usurped the nascent demon overlord Garm, in order to bring his plane of the Abyss under his control.
The ultimate purpose of these actions is unknown.
It is also believed that Cyphiax control the Red Orb of Dragonkind, and the Black Egg.