Chant of Resplendent Inferno

As you chant, a bright streak flashes at a point you (the origin caster) choose within range and then explodes outward. Each creature in a 15-foot-radius sphere centered on that point must make a Dexterity saving throw. A target takes 2d8 fire damage and 2d8 radiant damage on a failed save, or half as much damage on a successful one. The damage spreads around corners. It potentially ignites flammable objects in the area that aren’t being worn or carried.

This spell requires two actions to cast (or two rounds), and the second action can be fulfilled by another caster that knows this spell and has it prepared, or that uses a scroll of this spell. When the spell is cast in this manner, each caster expends the necessary spell slot to cast the spell. The damage done is increased by an additional 1d8 fire damage and 1d8 radiant damage.

In addition, for each spell slot above 2nd that each caster (if it applies) expends to cast this spell, the extra damage dice granted by having an additional caster assist, is multiplied by this combined number.

For example, both casters expending a spell slot of 2nd level causes a total of 3d8 fire damage and 3d8 radiant damage. If both casters expend a 4th level spell slot, the damage is greatly magnified for a total of 6d8 fire damage and 6d8 radiant damage. (i.e., Each caster spent 2 slots above 2nd level, and adding these together is 4. 4x1d8 is 4d8, added to the base of each 2d8 representing each damage type, we get 6d8 for each damage type).

If concentration is broken before the 2nd action requirement can be fulfilled either by the original caster, or a secondary caster, the spell fails and the slot(s) expended.