Relentless Endurance

When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead. You can’t use this feature again until you finish a long rest. Immediately after you use your Relentless Endurance trait, you can use your reaction to make one weapon attack.

Not killed outright means that the character’s hit point total was not reduced below 0 hp by more than half of their constitution score.