Deep

An unknowable entity that is associated with Ephemera in the same way Alios was associated with Aether and Balimor was associated with Deridah.
The Deep was contacted over a thousand years ago by The Feycourt when they created the The Feywild with its power. It also used Strahd von Zarovich by trapping him in an eternal cycle of heroes trying to strike him down while he in futility looked for a way to escape from the cycle and save his unrequited love. It is unknown if its use of Strahd was done consciously or not. 
The deep destroyed the world, but the fleeting bits of hope and humanity left managed to survive and destroy it, saving the world.