The Noxium Crystal is an evil crystal created by a villain named Dr. Natas. It hold all the weaknesses of the Superfriends and has the ability to kill them.
It was used by Natas but left the Legion of Doom and the crystal was sealed in the Hall of Justice. The Legion of Doom later got the crystal and used it on the Superfriends and threw it away. However, they were unaware they used it on their duplicates and the crystal was lost.