Colossal Coliseum

An immobile dome of shimmering, multicolored energy with a radius of up to 100 feet is summoned into existence around and above you and remains stationary for the duration. The spell ends if you leave its area.

The dome is composed of phlogiston, the material that makes up the inner spaces of the planes. The area within this sphere is for all intents and purposes a new plane, and can only be entered or exited through spells or magical effects that allow planar travel. Any creatures attempting to evoke such an effect against your wishes must succeed on a Wisdom saving throw against your spell save DC in order to successfully navigate the irregular phlogiston. On a failed save, they take 4d10 force damage. If this damage brings the creature to 0 hit points, they become petrified for the spell's duration. This effect also occurs if a creature is brought to 0 hit points while within 5 feet of the dome's walls.

The dome cannot be dispelled. If any portion of the walls take fire damage, that portion explodes violently, causing a chain reaction that deals 4d4 fire damage to anything within 10 feet of the walls, other than the walls themselves. If creatures succeed on a Dexterity saving throw against your spell save DC, they take half this damage. Each ten-foot square portion of the wall has a pool of 200 hit points that is immune to all damage aside from fire; if its hit points reach 0, it is destroyed, leading into the original plane that spell was cast on.