SRD:Vorpal

Vorpal: This potent and feared ability allows the weapon to sever the heads of those it strikes. Upon a roll of natural 20 (followed by a successful roll to confirm the critical hit), the weapon severs the opponent’s head (if it has one) from its body. Some creatures, such as many aberrations and all oozes, have no heads. Others, such as golems and undead creatures other than vampires, are not affected by the loss of their heads. Most other creatures, however, die when their heads are cut off. A vorpal weapon must be a slashing weapon. (If you roll this property randomly for an inappropriate weapon, reroll.)

Strong necromancy and transmutation; CL 18th; Craft Magical Arms and Armor, circle of death, keen edge; Price +5 bonus.