D&D 5e (2024)

Source: Arcana Unleashed

Ring, Uncommon (Requires Attunement)

The magical spike on the side of this ring allows you to sharpen your focus at a moment’s notice. When you fail a Constitution saving throw to maintain Concentration, you can roll up to two of your unexpended Hit Dice and add the total rolled to your save, potentially turning the failure into a success. The rolled Hit Dice are then expended.