User blog comment:Green Ninja/New Card-Jitsu Elements?/@comment-5257664-20130531020203/@comment-5257664-20130531034151

Wait, I have a better idea...
 * 1) Sensei's late father was given a special elixir that Tusk created to revive him, but it turned him evil. Once defeating him in Card-Jitsu Shadow (which Sensei actually teaches) he creates the Dojo of Darkness where he trains ninja the elements of Lightning and Hex. Or...
 * 2) Sensei had broken the forbidden elements off of the Amulet due to too many ninja turning evil after training them. When his brother came to visit, he reintroduced Shadow, Lightning and Hex.