Soul of Rebirth is an additional quest that places you in control of Scott, Josef, Ming Wu, and Richard, the four heroes who died during the main game. It's very short, composed of one town and two dungeons, which are just palette-swapped versions of Jade and Pandemonium, as well as a palette-swapped Tower of Mysidia where you can fight Ultima Weapon to acquire Ultima. But it does provide the story with some additional closure, I guess.