Alistair’s Otherworldly Arena

You attempt to transport yourself and a large or smaller creature you can see to an extra-dimensional space to do combat. The target creature must make a charisma saving throw or both of you are transported to the center of a demiplane arena 30 ft apart from each other. This arena is 120 ft in diameter and cannot be entered or exited by means of flight or teleportation and is immune to all damage. Both of you remain in this arena until one of you drops to zero hit points, in which both of you are transported back to the location where the spell was cast.