Boon of Spell Mastery, Variant (5e Epic Boon)
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Boon of Spell Mastery
Prerequisite: The ability to cast spells
Choose one spell you know of 1st-level. You can cast this spell at the lowest level, without spending a spell slot. In addition, you gain one 5th-level spell slot, that you can only use to cast the chosen spell at a higher level. Once you cast this spell at a higher level, you can't cast this spell again until you finish a long rest.
If you don't spend your 5th-level spell slot given by this boon, you can change the chosen 1st-level spell over the course of a long rest.