Vorpal Longsword
Melee weapon (martial, longsword), legendary, (requires attunement)

You gain a +3 bonus to attack and damage rolls made with this magic weapon. In addition, the weapon ignores resistance to slashing damage.

When you attack a creature that has at least one head with this weapon and roll a 20 on the attack roll, you cut off one of the creature's heads. The creature dies if it can't survive without the lost head. A creature is immune to this effect if it is immune to slashing damage, doesn't have or need a head, has legendary actions, or the DM decides that the creature is too big for its head to be cut off with this weapon. Such a creature instead takes an extra 6d8 slashing damage from the hit.

Versatile: This weapon can be used with one or two hands. A damage value in parentheses appears with the property–the damage when the weapon is used with two hands to make a melee attack.

Damage: 1d8+3 (1d10+3)
Damage Type: Slashing
Properties: Versatile
Weight: 3