Vorpal Sword

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 roll a 20, you cut off one of the
creature's heads. The creature dies if it can't
survive without the 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 creature is too big. It instead
takes an extra 6d8 slashing damage.