Titanfall

The prime deities grew tired of having their creations destroyed by the Titans, who roamed freely across the world. They knew that intervening directly and facing the Titans head-on would place an enormous strain in the world, possibly breaking it. So, they hatched a plan.
They would grow their base of worshippers and find a champion that would wield a fraction of their power, by crafting a series of artefacts imbued with a portion of the diety’s essence. These artefacts have been largely lost through time, and the few scholars who dedicate their lives to researching them have dubbed them the Relics of Titanfall
These relic-wielding champions battled the Titans, often gathering small armies from neighbouring tribes, in hard fought, guerrilla warfare-style series of battles. Thousands died during the few years this period of history lasted, which would later be called The Titanfall by the scholars who studied it.

Aftermath

The Titans were slowly defeated and broken, some forming entirely new landmarks due to their absolute gargantuan size.
Legends speak of surviving Titan or Titan-adjacent creatures that still sleep under mountains and in the depths of the oceans, such as the Tarrasque and the Kraken.
Later, lesser deities and other powerful entities across the planes have copied their older counterparts, and created relics themselves. While every bit as powerful in comparison, these may not be as lost as their more ancient counterparts, simply due to the fact that they were created more recently.