Dark Rejuvenation

As a bonus action, you may freely roll one of your hit die. If the next damaging spell you cast hits its target, then you heal the amount rolled on the hit die plus your proficiency modifier.

The effects of Dark Rejuvenation disappears at the end of the turn it was cast on if it goes unused.

You may roll an additional hit die upon reaching level 5 (2 hit die), and again at level 11 (3 hit die), and level 18 (4 hit die)