God-Sleep

The gods were nigh-immortal beings before they slew one another, but they were never invincible. God-Sleep was the method by which gods would separate their god-spirits from their physical forms and go into a state of waking sleep, allowing their god-spirit to continue acting while their physical body reconstituted.
It was God-Sleep that caused the Reckoning to drag on for so long, as gods would hurt one another and send each other into God-Sleep. Were Divinity not a finite resource, this could have caused the Reckoning to go on forever, long past the death of life and the destruction of reality: gods fighting over nothing, past the grip of time and space.
Thankfully, Divinity is a finite power, even if the gods had an almost limitless amount of it. Thus, once the gods ran out of enough Divinity to reconstitute their physical forms, their God-Sleep became permanent and their god-spirits were weakened.