Cage Match

This is the ultimate spell for any spellcaster who prioritizes awesomeness over effectiveness.

At the beginning of a battle, you bring down a metal cage around a creature you can see, and raise up a wrestling ring beneath it. The cage and ring are always to scale; for example, casting the spell on a Medium or Large humanoid creates an average-sized wrestling ring, while on a Huge fiend it would be the size of a city block, and greater still when cast on a Gargantuan. The cage is indestructible, and the ring will not buckle from being directly attacked.

When cast, you must choose a creature of equal or lesser challenge rating to the caged creature to conjure into the ring. This includes homebrew, duplicates, and creatures from other fictional universes, so long as the DM permits. This provides the two creatures a way to escape: The last one standing wins, causing the cage to lift and the wrestling ring to evaporate. You can conjure additional creatures into the cage as a bonus action at the end of each of your subsequent turns, and they will all be forced to fight each other.

You can continue adding creatures as long as their sizes do not eclipse the ring. The cage will raise in height to fit any creature too tall to fit as long as its width does not break the ring's parameters.

Powers like vanishing, portals, and sensory distortion can be used within the ring but cannot pass through it; the ring is entirely inescapable without defeating one's opponents in the cage match, and beings outside it cannot be attacked from within.

When a creature within the cage pulls off a legitimate wrestling move, the target of its attack is zapped by the cage walls for an extra 3d8 necrotic damage.