The Cross is the National Treasure of Park Mu-Bong of The Six. The R&D of this artifact, along with nano-technology, gives birth to God of High School's limiter, which can function like this national treasure.
It is a zen-beaded necklace with a yellow/gold cross hanging.
The artifact has the ability to store unlimited amount of Borrowed Power. The stored energy can be used for a one-time power-up.