Axe of the Furyborn (5e Equipment)
From D&D Wiki
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.