Bonded siphon

As an action, fire a thin green-black beam of necrotic energy at a creature you can see. Make a spell attack, if you hit, you create a visible necrotic tether between yourself and the target. You deal 10% of the creatures maximum health as necrotic damage and are healed for the value of the damage dealt. 

At the start of each of your turns while the spell lasts the effect repeats automatically dealing necrotic damage and healing you for an equivalent amount. 

The bond is severed if the target hits you with a spell or weapon attack, if you cast this spell again, or if length of the tether exceeds 300 feet