Seithr oil was a liquid substance taken from the petals of the Seithr plant, which grew on a small island in the frigid northern seas. The oil was normally harmless and mainly used for preserving pearls, but when a certain incantation was spoken over it, along with the offering of a blood sacrifice, it became a deadly, caustic poison. It ate away anything living or previously alive, if the poisonous effect was to be stopped the wound is stated in "Eragon (book)" to never regenerate or heal even with the help of the Deities. Strangely seithr oil left everything else with which it came in contact untouched. Because of this, it was frequently used by assassins and Ra'zac.