Entombing of the Titans

"Ultimately the Titans could not be vanquished and the only way they could be stopped was by magically entombing them in ice, an act which brought an end to their terrible reign."

- The Titans' downfall being mentioned in the Halliwells' Book of Shadows.

The entombing of the Titans was an event that occurred during the days of Ancient Greece. After seeing the immense damage that Cronus, Demitrius and Meta inflicted on the world, the Elders used an ancient vapour to turn several mortals into gods. They proceeded to do battle with the Titans, and finally, upon seeing that they could not be vanquished at their hands, they magically entombed them in ice, bringing an end to their reign.

Background
During the days of Ancient Greece, the three Titans, Cronus, Demitrius and Meta, ruthlessly ruled the earth, forcing mortals to worship them. Being as powerful as they were, almost god-like, none dared oppose them until, at long last, the Elders used an ancient vapour to turn several mortals, including Zeus, Gaea, Athena, Aphrodite and Mercury, into gods in order to defeat the Titans.

The entombing
However, the gods were unable to vanquish them, and, instead, used magic to seal them in tombs of ice. This ended their evil reign on the earth, but the gods then proceeded to force mortals to worship them just as the Titans had done before after their power went to their heads. The Elders presumably removed the vapour from them afterwards.

Behind the scenes

 * This event was mentioned in Oh My Goddess! Part 1 and Part 2.