Dark One's Balance

"You may share my gifts with your compatriots, but only under my terms. Are my gifts not sufficient for you little one?"

When you have temporary hit points granted by Dark One's Blessing,  and you heal a creature, your healing increases equal to your Charisma modifier + your warlock level (minimum of 1), and you take half that much damage. Also, any healing you receive from another creature first reduces your temporary hit points, and then heals you for that much less.