The Black Crown is a crown made of bone obtained from the body of a demi-god. There's a black jewel at the front.

This artifact serves two purposes: it's the Lich phylactery of the Black Crown (the individual) and the world stone binding the Archipelago together. More details on it's history can be found here.

This artifact must be destroyed in order to truly defeat the Black Crown. Without destroying it, he can always come back. However, it's also a WMD set to destroy the entire plane of existence the PCs call home. They'll be faced with the impossible choice of either killing the villain or allowing their home world to remain untouched.

None of these can be called upon by anyone other than the Black Crown.

• It's his phylactery, allowing him to possess the body of anyone nearby, as with a normal Lich.
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• The Black Crown can never leave his demi-plane home, due to this item being tied so closely to the existence of the plane.
• Destroying this item transforms the setting into an Ethereal Pain campaign. However, this should be a slow process, over the course of several months, allowing wizards in the know to create many stable areas. With advance warning, the entirety of Arcanus could be made stable, but most other islands will be lost. The casualties would be greater than 99% of the population of the Archipelago.
• Injuring the Black Crown while he's wearing this will cause the local area to become unstable, similar to the Tatters.

• Convince the Black Crown to give up on living. He knows how to disentangle the world stone from this item. He's the only one that can.
• Contact the divine enemies of the Black Crown and ask them to destroy it. They will send and angel with the capacity to accomplish this task.
• Sora, if in the form of The Crusader, has the ability to erase this item from existence. This will also instantly end her existence. However, this will first require destroying the Black Crown's body, which is no easy feat in and of itself. It's quite likely that The Crusader's energy will be spent before this is possible.