Unyielding Warrior (5e Feat)
From D&D Wiki
Prerequisites: Constitution 13 or higher
When you would be reduced to less than 1 hit point, as a reaction, you may grit through the pain for a short while. When you do, you remain conscious regardless of your hit point value until the end of your next turn. After your next turn ends, your hit points become 0 and you drop unconcious. Once you use this feature, you may not use it again until you finish a long rest.