Skullsoul

This grim, alabaster figurine is the size of a human skull with an elaborately sculpted base. The Skullsoul is inert until it is bonded with a living creature. It then becomes a powerful and never-questioning companion. The Skullsoul functions as a demilich without the devour soul, rejuvenation, telekinetic storm, and torpor abilities, and with a hardness of 10. It is under the complete control of whoever bonds with the skull. Bonding with the Skullsoul is a ritual that takes 24 hours to perform, and doing so is an evil act. Paladins and good clerics who do so become ex-paladins and ex-clerics respectively. If they wish to atone, they must intercede with their deities (expending 2,500 gp in rare incense and offerings) and must promise to contain the skull.

The Skullsoul cannot be destroyed but it can be contained. As long as the Skullsoul stays pierced by either a weapon with the vorpal special ability or a holy avenger, it is contained and entirely inert until the sword is removed.