Exodia was, at first, a beast of such incalculable and intimidating might that he was split up into five distinctive parts, each one of them chained and sealed away by magic from all the other monsters, hence the Forbidden One. That would have prevented his power from ever being used again.
Card Game
There are five distinct Forbidden One cards, each one symbolizing a part of the complete Exodia. In order for Exodia to regain his tremendous power, all of the five cards must be reunited in the player's hand. This brings the parts of the Forbidden One together, allowing Exodia to break free, his strength being so great that he guarantees the player an automatic win, regardless of the situation.