Blade of the Burnt Hand (5e Equipment)
From D&D Wiki
Weapon (longsword), legendary (requires attunement)
An ancient, katana-esque longsword, absent of decorations and stored in a plain-looking but fireproof and absurdly strong wooden sheath.
While not attuned to this magic weapon, you have a -2 penalty to attack rolls and damage rolls made with it; while attuned to it, you have a bonus of +2 to both. You can only become attuned to the weapon once you have hit an enemy with it in three different combats. After you become attuned to it, the Blade deals 4 fire damage to you the next time you unsheathe it, permanently scarring your dominant hand; this fire damage ignores damage resistances, but if you are immune to fire damage, the Blade will not attune to you. Once burned, any attack rolls you make with that hand without using the Blade have disadvantage. This effect can be removed by a remove curse spell, but only if you choose to break your attunement with the Blade. While you are attuned to the Blade, whenever you unsheathe it, melee attacks with it deal an extra 3d8 fire damage until the end of your next turn.
The sheath can be used as an improvised club. While holding the sheath, you have resistance to fire damage regardless of whether or not you are attuned to the Blade.