Old Omniverse

"There was an old omniverse that existed before our Prime Earth was created. The Anti-Monitor destroyed it but a new one was created. We have to stop the coming Apocalypse because after our Earth, Savitar will crave more power and destroy the new omniverse as we know it." - Xneen.

Description
The Old Omniverse contained multiple Marvel and DC universes. Doctor Manhattan broke the boundary between universes which led to the events of the Unity Trilogy. Manhattan was defeated but a Crisis loomed years later, and The Flash along with The Monitor assembled a multiversal team of heroes to combat the Anti-Monitor. They failed but creating a new omniverse was their saving grace, thus we have the New Omniverse which contains Prime Earth.