You are battle-hardened and resilient, fighting to your last breath while shrugging off mortal injury. This grants you the following benefits.

• Increase your Constitution score by 1, to a maximum of 20.
• If you are reduced to 0 hit points (and not killed outright due to massive damage), you can use a reaction to make a Last Stand before falling unconscious: make a single melee or ranged attack. If your attack is made against the creature responsible for dropping your hit points to 0, you have advantage on the attack roll and critically strike with a roll of 19 or 20.
• Once you use this ability, you cannot use it again until you have completed a long rest.