This spell comes from the Dark Talisman that you can get along the way. After you defeat Songi in the Seru-kai, go to the West Voz Forest. Go back to the dead Genesis Tree. Near the stump there will be a chest containing the Dark Stone. Take the stone to Zalan the Jewler of Jeremi. He will make it into the Dark Talisman. Equip this on any of the characters (Noa is recommended). It allows the character to cast the deadly Jedo spell. It also increases Dark Power protection. This is effective against all enemies. It works the best with field enemies and the monsters of the Bio Castle.