Worldbuilding Challenge Day 8 - Biggest Secret

What is your world's biggest secret?

The world of Eodia is actually in a 'demiplane' of existence, transported there by the Caeruleans. The Caeruleans were once rulers of an entire plane of existence, but a revolution nearly drove them to extinction. As their enemies were closing in the Caeruleans unleashed their final weapon, the Leviathans. The Leviathans were nigh indestructible killing machines, programmed to destroy all traces of sentient life and then self-destruct. The Caeruleans then opened a wormhole and transported their last remaining city to another plane, planning to one day return to their home when the Leviathans had achieved their directive. Yet the Leviathans had a flaw in their code, and thus after wiping out all sentient life they set out across the empty void to continue their rampage in other planes.

On the day of The Ending the Caeruleans discovered that the Leviathans were closing in on their new homeworld, and once again attempted to teleport their civilization away to safety. This time however the wormhole the Caeruleans opened became unstable, and the world of Eodia was sucked into a demiplane. It exists now in a place 'between' planes. The Void Sea is actually the bottom of the wormhole and the light in the sky is from the destination star. The Leviathans in their great power are able to enter the Void Sea, yet cannot escape it's pull to fully enter the 'world' that Eodia now inhibits. If the day were ever to come when the Leviathans could rise from the dark seas, the complete destruction of Eodia would soon follow.