Relentless Rage is a Barbarian feature gained at level 11 in the 2024 SRD. Your Rage can keep you fighting despite grievous wounds.
Your Rage can keep you fighting despite grievous wounds. If you drop to 0 Hit Points while your Rage is active and don't die outright, you can make a DC 10 Constitution saving throw. If you succeed, your Hit Points instead change to a number equal to twice your Barbarian level.
Each time you use this feature after the first, the DC increases by 5. When you finish a Short or Long Rest, the DC resets to 10.