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 choose to return to the Overworld from the Underworld after kill 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 easy 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 become easier.

Another method is using Astral Decay.