Axe of the Furyborn (5e Equipment)

Weapon (battleaxe), legendary (requires attunement)

A crude looking axe with a cutting edge made of sharp obsidian. When you attack with this weapon and you roll a natural 20, you may take a bonus action to immediately make another attack. Whenever your attack hits a target, you may add 1 to all future damage rolls with this weapon against that target.
You lose the damage bonus with this weapon when the target dies, you use the weapon to attack a different target, or 10 minutes pass.