You attain the pinnacle of resilience in battle, giving you these benefits.
Defy Death. You have Advantage on Death Saving Throws. Moreover, when you roll 18–20 on a Death Saving Throw, you gain the benefit of rolling a 20 on it.
Heroic Rally. At the start of each of your turns, you regain Hit Points equal to 5 plus your Constitution modifier if you are Bloodied and have at least 1 Hit Point.