There was a rumor that supposedly if you gathered all 12 52 Soldier items and the Masamune in Golden Saucer, there was a way to "clone" Sephrioth in Hojo's lab in the Shinra Mansion. But imo it was bull crap. And I have yet to find a way to make it work, or for that matter anything on anyone actually doing it. Kinda like the Ambulance Ending in Silent Hill.