Divine Intervention

When a friendly creature within 60ft of you drops below 0 hit points, and not outrightly killed. Casting "Divine Intervention" will sacrifice your hit points, dropping you to 0, but stable (don't make death saves) and returning the exact amount of hit point you sacrificed to the target. If the number of hit points you sacrificed is greater than the max of the target, the target gains the extras as temporary hit points. 

If cast at a 6th level, the spell can instead be cast as a reaction.