Vorpal Sword
Weapon (any sword that deals slashing damage), legendary, requires attunement · 2014 rules, SRD 5.1
+3 attack and damage; ignores resistance to slashing. On a natural 20 vs a creature with a head, you cut it off (killing it unless it doesn't need it); otherwise an extra 6d8 slashing.
Also in the 2024 rules (SRD 5.2).