There have been multiple Fusion Stones but all of them were destroyed, and the last one was found by Circe. She had been looking for it for centuries, and when she did find it, she stated that she found it outside of the Universe.

The Fusion Stone grants the wielder high level of powers. Circe wanted to test this out, so she found a desperate boy named David Reid, she tested it, and David got high power levels to the point where he was capable of defeating Superman and Wonder Woman.