Spirit annihilation was a divine magic spell available to priests of Bane, the Black Hand. It completely destroyed the spirit of a dying or recently dead creature so it could never return or reveal information via speak with dead. After Bane's supposed death in the Year of Shadows, 1358 DR, Cyric granted spells to Banite priests for a decade, so it was possible this spell could be found within the church of Cyric. Otherwise, the only sources for this spell were scrolls penned in previous centuries and hidden in former Banite centers of power.
This spell effectively removed a spirit from all planes of existence, preventing it from reaching the Outer Planes and the afterlife. It was primarily used by priests of Bane on their victims as they were dying or shortly after death to prevent them from being questioned by speak with dead spells. The victim's spirit got one chance to resist destruction or else it was forever lost, unable to be queried, raised, reincarnated, or resurrected.
The casting cleric had to touch the body of the victim while casting this spell and the victim had to be near death or dead for only a few minutes. After that, the spirit escaped to the afterlife and could not be attacked by this spell. The force of this spell utterly destroyed a spirit's identity, memories, and personality, but its raw energy could theoretically be channeled into some other evil use. The body remained unchanged and could still be used to create a mindless undead creature.
This spell required only verbal and somatic components, including touching the body of the victim while the spell was being cast.