Desperation

At the uttermost end of need, you channel the inner life force of a creature to cause a surge of natural healing within it, but at a steep price.

The target must be a willing creature and may spend one of its Hit Dice to recover a number of hit points equal to the amount rolled. However, the creature suffers one level of exhaustion, cumulative with prior levels it has already sustained.

A creature healed by this spell can spend one additional Hit Die when you reach 5th level (for a total of 2 HD), 11th level (3 HD), and 17th level (4 HD).