Glory Kill

Whenever you attack a creature that is Stunned, Prone, Paralyzed or Frightened of you with a melee attack and hit. You may, as a bonus action, add 2d8 of the attacks damage type to your damage roll. If you kill a creature this way, you regain a number of hit points equal to the additional damage you dealt using this feature. You can use this feature up to a number of times equal to 1 plus your strength modifier. You regain all uses of this feature after a long rest.

The number of additional dice you roll increases to 3d8 at 5th level, 4d8 at 10th level, 5d8 at 15th level, and 6d8 at 20th level