Draining Thread

You open a wound on your body, shooting out a thread of blood between yourself and a creature within range. As you cast the spell, you must spend a number of hit points. The creature must then succeed on a Dexterity saving throw or take 7d6 + the number of hit points spent as necrotic damage. If this damage brings the target to 0 hit points, half the hit point cost is refunded as blood drains through the thread back into your body.   

At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, the damage increases by 2d6 for each slot level above 4th.