Curse of Shared Pain

Choose a willing target within range and one other target within range. The secondary target can make a CHA save to negate the spell. Thereafter, any damage the willing target takes, inflicts twice that much damage on the secondary target, of the same type. The secondary target can repeat this save at the end of each of its turns, ending the spell on a success. 

When cast using a spell slot of 3rd level or higher, you can target one additional secondary target within range for each spell level above 3rd.