Vorpal sword

 is a greatsword, longsword or Scimitar that have the ability to behead opponents on lucky strikes.

Description
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 GM 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.

History
Derrig, attending the wedding of Vex and Percy, had Vorpal longsword. However, Liam did not roll 20 on his attack roll to decapitate an opponent.

Trivia
Vorpal sword is based on phrases in Lewis Carroll's poem "Jabberwocky".