Betty's Spell's Empowerment

You imbue a willing spell caster creature with shadowfell's magic. While under the spells effects the target's spell attacks, which must require an attack roll, deal 1d4 extra damage of the same type of the spell used.
 If the target of this spell reduces an enemy to 0 hit points using a spell attack which required an attack roll, until the end of the next turn the target can choose to reroll one damage die of it's next spell attack, if it is done the new dice must be used.

At Higher Levels: When you cast this spell using a spell slot of 3rd or 4th level, the bonus to the extra damage increases to 2d4. Using a spell slot of 5th, 6th or 7th level , the bonus to the extra damage increases to 3d4. At last using a 8th or 9th spell slot the extra damage increases to 4d4.