Single Spell (5e Epic Boon)
From D&D Wiki
You have chosen to dedicate all of your magic to a single, glorious spell. When you cast a spell that requires a spell slot to cast, that has the instantaneous duration and require either an attack roll to hit or a saving throw from your target, you can consume all your spell slots, dealing additional damage equal to 1d8 for each spell slot level you spent. For example, consuming two 5th-level spell slots will increase the damage in 10d8. If the spell targets multiple opponents, you must choose one to suffer the additional damage, or spread the additional damage between the multiple targets.