Whereas Man of Steel saw Superman put through his paces, it didnt feature the famed green rock known as Kryptonite. Its expected that Kryptonite will play a part in next years Batman V Superman: Dawn of Justice, but the old green stuff has already turned up in Supergirl. Its clearly Karas first experience with Kryptonite, and we even get to see a Kryptonite knife used on Astra towards the end of the episode. With Kryptonite now a thing in the world of Supergirl, this deadly rock could have a huge role to play as the show progresses.