Syphon Vitality

You momentary take on a vampiric form to lash out at a creature within reach. Make a melee spell attack against the target. On a hit the target takes Necrotic damage equal to 2d10 plus your spellcasting ability modifier and you gain Temporary Hit Points equal to half the amount of damage dealt.

Using a Higher-Level Spell Slot. The damage increases by 2d10 for each spell slot level above 1.