Armor of unyelding fury (5e Equipment)
From D&D Wiki
Armor (hide), legendary (requires attunement)
This crude armor is made of thick blood stained furs and pelts held together with thick ropes, chains and leather straps. While you are below half of your maximum hit points rounded down, you add your proficiency bonus to all of your saving throws. In addition, whenever you are reduced to 0 hit points and not killed outright, you may instantly take a turn within the current initiative step, however you may not use the turn to heal yourself in any way.