Stalwart Guardian (5e Feat)
From D&D Wiki
You know that what awaits what is important to you is a dangerous fate if you lose this battle. The sparkle in your eyes ensures that in a last-ditch effort, you'll take down your opponent with you.
If you take damage that reduces you to 0 hit points, you can use your reaction to delay falling unconscious, and you can immediately take an extra turn. While you have 0 hit points during that extra turn, taking damage causes death saving throw failures as normal, and three death saving throw failures can still kill you. When the extra turn ends, you fall unconscious if you still have 0 hit points.
Once you use this feature, you can use it again after finishing a long rest.