Persistent Warrior (5e Feat)
From D&D Wiki
Prerequisites: Constitution of 15 or more
You will not go down by the strikes of your enemies easily, you will persist until your last breath.
Whenever you would be reduced to 0 hit points, you can make a Constitution saving throw with a DC of 10 plus half of the damage taken (rounded down), this number cannot exceed 26. On a success, you are reduced to 1 hit point instead. On a fail, you are reduced to 0 hit points and have disadvantage on your first death saving throw. You can only use this feature once per long rest.