Debt Collector

When you lend your power, it comes with a hefty cost for those who dare accept it. You gain the following abilities:

Cursed Loan: As a  bonus action, you can grant an ally within 30 feet a powerful boon. The ally gains advantage on their next attack roll, saving throw, or ability check, and the roll gains an additional bonus equal to your Charisma modifier (minimum of +1).
This bonus lasts until the end of their next turn.

Debt Collection: at the start of their turn immediately after being granted the boon (regardless of usage), they take necrotic damage equal to half your Charisma modifier & warlock level combined (minimum of 1). This damage cannot be mitigated.

You can use this  feature a number of times equal to your Charisma modifier, and you regain all expended uses after a long rest.