Empyrean
The Empyrean, or the Warp, or the Great Ocean is a realm of raw unchecked psychic energies fed by mortals' emotions, thoughts, desires and vices. The Empyrean is home to the Daemons: swilring masses of enemies, which incarnate into a random, mostly vile, form when entering reality. These are what we call Daemons.
Psykers, however, also use the warp to power their psychic abilities, thus forming a clear beacon in the Empyrean. Those not trained to dimish this aetherial light, will fall victim to a daemonic entity which manifests into the body of the host, and twists it to it's liking. This is called mutation, and mutants are heresy, unless otherwise edicted.
Though the Warp is what the Emperor tried to release humanity from until his demise, the Empyrean is our greatest ally and greatest foe. On the one hand, it enables humanity to travel the stars and communicate throughout the galaxy. But also, it is home to the Chaos Gods which constantly try to thwart the Emperor and his plans.