Blood Siphon

Choose a target in range that you can see.  You must expend 1 hitpoint to cast this spell. The target must succeed a constitution save based on your spell save DC. On a failed save, the target takes (2d10) + your intelligence modifier force damage and you heal for half of the damage the target takes. On a successful save, the target takes half damage and you do not heal. Subsequent uses of this spell doubles the amount of hitpoints per target you must expend to cast it. This resets after a long rest.