The Overworld is the cosmic plane where Gods and Titans live. Subject to permanent changes due to the Overworld War between Gods and Titans or due to Titans absorbing and waging war on each other, the Overworld is infinite in both space and time. Its history is eternal and there are no known origin point for its existence. It just is.
Once a global entity called the Great Adversary, formed of all the natural concepts merged together, it was defeated by the Primarchs and torn into pieces, thus forming the Overworld known today. After the First Titanomachy, the World - merged with the Overworld until then - was separated from it, creating the Otherworlds in the process.