The Underworld is not a myth. It is a place one ruled over by Sakure and other Titans. With Sakure dead it has left those in the Underworld in a state of limbo. Not able to return to the land of the living and not able to ascend or descend to heaven and hell.

To this end a lone rider was sent out through a tiny crack in the door that locked the Underworld. This rider made it to the lower level of the Death Star Cavern and is looking for you.

If you can find them you can access the Underworld and help right some things!

The Underworld is the final final endgame realm. The realm to beat all realms! It is where you go once Impact Prime is done with and it is where you will go with a severe can of ouchies!