Soul Leech

To borrow Morgantha's heartstone for a period of three days, the players must promise to undertake a particular task for her. As a form of insurance to ensure their compliance, Morgantha takes their dreams as collateral once the players accept her offer.
Morgantha warns the players that, if they fail to complete her task within fourteen days—or if they betray her—she will immediately devour their dreams, preventing them from gaining the benefits of a long rest forever.
If the players complete Morgantha's task within the allotted time, however, she will release their dreams and allow them to return to their owners. In either case, you can take long rests normally until the fourteen days have elapsed.
Morgantha’s task is simple. The players must deliver a soul leech—a tiny fiend resembling a leech with ghost-white, translucent flesh and purple-blue veins, and which Morgantha has imprisoned in a corked glass vial—to the office of Strahd’s chamberlain, Rahadin. Upon arriving in Rahadin’s office, you must uncork the vial and allow the leech to attach itself to the underside of his desk, where it will render itself invisible and virtually undetectable.