Sacrifice

Achieved by sacrificing yourself to the Demon King in order for him to save the Overworld from the Sky Eater.

You can get this ending by:
 * 1) Activate all 5 monoliths
 * 2) Enter Hell through the stone arch
 * 3) Blow out the 3 blue torches in Hell
 * 4) Head to the Demon King's throne and talk to him
 * 5) Choose to sacrifice yourself to him