Colocasia Root, also known as Elephant Ear or Taro, is a plant naitive to the South Pacific. In the episode Witness, the Witch Hunter Eben claimed that the presence of a powdered form of the plant would allow him to kill the witch John Blackwell. His plan was foiled by Blackwell's use of a Medallion which overwhelmed the effect of the plant.
In Return, Eben once again used this substance, refered to as "Colocasia Powder", to block Cassie's powers during her capture by the Witch Hunters.