A Leviathan is Slain in the Ocean

As the Leviathans began to spread throughout Earth's oceans, they began wreaking havoc on the native ecosystems. They killed off numerous species, and caused unnaturally-sized floods to batter the coastlines. Humans were also threatened. Soon, the Watchers created their own storm to kill the largest Leviathan before he could destroy anything more.

Soon after the Leviathans were set loose in the real world's oceans, they began hunting the wildlife that proliferated during the time. Some areas of the ocean also became crowded due to the Leviathan's unnatural sizes. The largest Leviathan was centered around the Mediterranean Sea, in sight of some of the Proto-Indo-Europeans.

Over time, the Watchers realized how destructive the Leviathans were being, and planned to stop them. With the Gatekeepers' help, some of the Leviathans were successfully transported back into the realm, but the largest remained where it was. The Watchers then decided to slay him, by creating a massive storm over his residence, and striking him with lightning until he went still.

It is thought that a group of Proto-Indo-Europeans witnessed this event, and believed the storm was caused by a deity from the sky. Afterwards, they then adapted the event into their own beliefs, and as Indo-European cultures diversified, the story was changed to fit their respective pantheons. Most notably, there was the story of the biblical Leviathan.