Blood Drain

One ally within your sight must make a Constitution saving throw, or be drained of 1d8 hit points per your round. You will gain back whatever life is stolen from your ally.

Your ally has the option to willingly fail their Constitution save. 

On a bonus action after a failed saving throw, the caster may steal 1d4 worth of hit points to recover a level one spell slot, stacking d4 at higher levels for higher spell slots.