Vorpal Greatsword

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 atleast one head and roll a 20 on the attack roll, you cut off one of the creature's heads. A creature is immune to this effect if it is immune to slashing damage, doesn't have or need a head, or the GM decides that the creature is too big. Such a creature takes 6d8 slashing damage instead.