Alchemy: Kamigawa

Molten Impact

Molten Impact deals 4 damage to target creature or planeswalker. If excess damage was dealt this way, when you cast your next instant or sorcery spell, Molten Impact deals damage equal to the excess to target creature or planeswalker.

This sorcery deals 4 damage to target creature or planeswalker. If excess damage was dealt this way, note that excess damage, then you get a one-time boon with "When you cast an instant or sorcery spell, this boon deals damage equal to the noted number to target creature or planeswalker an opponent controls."