Arterial Tap

Make a melee (grapple) spell attack against a creature you can reach. Upon a successful grapple, the target takes 2d10 necrotic damage and is restrained by you. The target may retry its save on each of its turns. Each failed save deals an additional 1d10 damage. If a creature is dropped to 0 hit points in this manner, the caster may roll hit dice equal to the number of rounds the creature was restrained. 

At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, the initial damage increases by 1d10 for each slot level above 1st.