Each of the Great Powers possess a personal magical box artifact.
The Great Powers are beings of immense power, so powerful that not much can ever harm them and they also live forever.
These box artifacts serve as a way for these beings to die. A way of committing a sort of suicide if they ever get tired of life. The Great Power willingly enters their box artifacts and will cease to exist. It is theorised that they pass on to an afterlife of sorts. It is shown that a Great Power can be killed by another's box artifact when Mister Dark was pulled into The North Wind's artifact.
Each of the Great Powers have their own unique artifacts.
Hope - The Pandoran Jar
The North Wind - The Cask Of Ancient Winds
Mister Dark - The Vault of Primal Darkness
These artifacts are kept safe in hidden otherworldly places that can only be accessed by their owners.
The idea of creating the box used by the Boxing League to imprison Mister Dark was copied from these Box Artifacts.