Sky Eater

The Sky Eater will inevitably fight the player when the Shadow envelops the entirety of the land (3 days), or when the player chooses to return to the Overworld from the Underworld after killing the Demon King.

The Sky Eater has 9999 Health and is practically unbeatable.

It summons several Shadow enemies which deal a significant amount of damage.

Boss Fight
There are some possible methods to kill the Sky Eater.

Player can go to the Underworld and kill the Demon King, then choose to return to Overworld with an incredible buff. That makes the final fight with the Sky Eater easier.

Another method is to use Astral Decay, either by learning it, or choosing to play as the Ascended.