The Reckoning

The Reckoning was a world-ending event in which Valimos, The Time Lord, who was the leader of the Old Gods, erased himself from the timeline via an infinite time loop, causing the timeline to reset and the other Old Gods to lose all their powers.

History
Valimos, The Time Lord had grown tired from seeing the other Old Gods be complacent to the events that happen in the Mortal Realms. He then tried to absorb time onto himself and created an infinite time loop where only he could exist, causing the timeline to reset and the Old Gods losing their powers and abilities.

The Old Gods were then forced to fight among each other to maintain a strong base of worshipers. After a grueling 10 days of endless war, the surviving six agreed to hold power over one domain and never to interfere with the ascension of another Old God before splitting up.

Origins of The Gemless Sea
This event was briefly mentioned by Morgana when the party met her in Mizukasa, where she mentioned she had recovered most of her godly powers since.