Nexus is the living embodiment of what was before TOAA created everything, pure darkness. He is a failed experiment, TOAA knew that it was not lively enough for an eternity. So he exiled the darkness to a realm darker than anything imaginable. Blacker than death itself. All that can be heard is the wails of lost souls and pure darkness. Right before the first big bang many trillions of years back predating the dimensions before, the darkness formed into a being. One being made of the all the lost souls in one, and all the power of the darkness harnessed in it. It escaped the realm, but soon learned the realm was his. He waited for TOAA to create the first life and planets so he could begin consuming them in darkness. He is the reason space besides the planets and stars is a black void. TOAA knew it was unreversable and chose this as the fate of the current set of dimensions, that one day Nexus would engulf everything in darkness. He is a being who chooses not to involve himself in the affairs of the living and cosmic, but one who slowly eats up everything. He has only involved himself when his interests where threatened, like when Galactus tried to consume Planet Earth and nothing could be done. He simply consumed Galactus and teleported him out in the 5th dimension and left. As Earth is the epicenter of the everything and will be the last thing he finally destroys. Or when The Living Tribunal wanted to make the Universe a beacon of light, Nexus combated The Living Tribunal, it was power The Living Tribunal did not know of, until TOAA convinced Nexus to let him out, he had The Living Tribunal hostage in his home in the dark. Or when the Beyonder accidently tried to gain entrance to Nexus's home, Nexus destroyed Beyonder and recreated him so he had no recollection of what happened. But Beyonder would always be tainted with the remnants of his dark touch.
Power Level: Nigh Omnipitent