Unholy Fervor

A willing creature you can see within range is imbued with unnatural vigor. That creature gains Temporary Hit Points equal to 2d8 + your Spellcasting Ability Modifier. If the creature hits a target with a weapon attack while it has these hit points, it can choose to expend all of them to deal additional Necrotic damage equal to the number of Temporary Hit Points it expends.

Using a Higher-Level Spell Slot. You can target one additional creature per level of spell slot above 2.