Crashing. Banging. All around portals opened up into an unseen void, spewing forth chunks of land and various gigantic pieces of matter. As if subject to a fluctuating force, they stop and start, plummeting into some semblance of order. Various gravities and atmospheric pressures boggle together and meld. With a static charge, an acceptable level of friction buzzes into existence. And as a din and a racket clatter across, a new dimension fizzles into life.
From a rift in the newborn sky, a mountain steadily drops and makes an enormous splash in the sea below, sending flames and sparks crashing outwards. It shifts over and collides with another landmass, anchoring it in place. Phasing into reality, a fully-developed castle, complete with bits of swarms of bats, materializes elsewhere. Freshwater and saltwater, with all their variations of salinity, swirl together in a menagerie of seas. Meanwhile, in other already established dimensions, towns and villages disappear into clefts of blackness. Befuddled townspeople wonder what could possibly be going on as pieces of the world they know slip out of their reality.
A certain divine guilty party, wielding a dimension-disrupting sword, shuffles his feet back and forth for a few moments before mumbling, "Oops."
Meanwhile, in the ever-filling cleft between dimensions, a new world is forming out of parts of the old ones. Landmarks from various realities and universes flood into the landscape. Noticing an excellent opportunity to certify their power, would-be gods enter the realm, establishing it as their own to rule and develop. Organizing the cacophony into a respectable microcosm, the immortals create order through clever schematization and divine prowess. But being immortals, these pioneers desired more from their makeshift dimension. Prodding at a wormhole floating above the city of Truce, one of them creates a static gateway from reality to the cleft of dimensions, allowing people to enter as they please. With help from new adventurers entering the cleft, a new dimension achieve greatness.