The Evil Crystal is a massive green colored gemstone that has a number of strange powers and abilities. Hidden inside some ancient ruins on Third Earth, the crystal has the power to not only make phone calls but also to teleport people.
During their exploring trip, Lion-O, WilyKit, and WilyKat stumble upon the crystal. They immediately start using it to make prank calls to Mumm-Ra. The evil mummy then teleports to the ThunderCats location and reveals that he had been searching for the crystal for centuries as it can restore him to full power.
When Mumm-Ra first throws the ThunderKittens and then Lion-O into the crystal, it teleports them to an unknown location where there exists an equally sized pink crystal as well as a manual on how to operate the crystals.