Watts' Spare the Wounded

You see a living creature get reduced to 0 hit points. As a reaction you can pray upon the creature and they become stable as you channel your divine vitality into them, causing a surge of intrinsic healing. You spend one of your Hit Dice and the creature drops to 1 hit point instead. 

A creature touched by this spell can spend one or more of their own Hit Die when you reach 5th level (2 hit die), 11th level (3 hit die), and 17th level (4 hit die).  For each Hit Die spent in this way, the player rolls the die and adds the character’s Constitution modifier to it.