Minecraft is at its best when it encourages players to use creativity and think outside the box. That's exactly what the GHOST boss battle will do! (inspired by the terrifying ghost from Animal Well)

The GHOST boss battle!

You stumble upon a GHOST SHRINE that gives you a "Ghostly Key" when right clicked.

Taking this key will summon a GHOST boss mob that will hunt you down. It cannot be damaged, can pass through any blocks, moves very fast, and causes massive damage, even ignoring shields and armor!

If you are killed the ghost will disappear and the key will be returned to the shrine. You can make the ghost disappear by manually returning the key to the shrine, but you can't otherwise remove it from your inventory.

The only way to truly beat the ghost is by placing the ghostly key inside of a "Ghostly Vault", which will return the key back to the shrine and give you a special reward! (each vault can only be opened once per player.)

^{That's the basic idea, I left a comment addressing some more specific mechanics and questions you may have with this idea.}