On the last day of the world, the Soulsong will be held in the Order's great cathedral to purify and put to rest the souls of the dead. This rite requires the holy clavis, which draws the souls of the dead to it.
Only the Soulsong can ease the suffering of the hundreds of millions who wander within the Chaos, tormented eternally in the darkness. And only Vanille, the saint who can hear their voices, can perform it. The Order must have both to perform this final rite.
I can't wait to see that ceremony, can you? All those useless souls purified forever in return for Vanille's life... Even the soul that was once Serah. Wiped away like it never existed.
The ritual is proposed by the god Bhunivelze to his devotees, the Order of Salvation, to grant them eternal bliss in the new world. The ritual destroys the souls of the dead so the living would forget their existence, enabling them to start over with a "clean slate," freed of their burdens and weaknesses. The Order, believing that being chosen for rebirth in the new world is an honor, come to believe in carrying out the Soulsong.
Two things are needed for the performance of the Soulsong: a sacrificial 'saint' who can hear the voices of the dead and call out to them, and the holy clavis, a powerful artifact forged by the gods for the express purpose of summoning the dead and destroying them. The Order has dubbed Vanille as the saint, having gained the power to hear the dead, and the clavis is hidden in the ruins of the Dead Dunes.
The Soulsong was never about bringing peace to the dead. It was about making things easier for the survivors. If they succeed in destroying all those souls, then the living would forget they ever existed. The Order wants to cut them away like a tumor. Then we'll be free from the dead weight of the people we used to love.
Thirteen years before Lightning awoke from her crystal stasis, Vanille and Fang awoke from theirs in Luxerion. They were found by Lumina and taken in by the Order, housed within the Luxerion Cathedral. Upon discovering Vanille's new power, the Order became interested in her and placed her under their protection and revered her has a saint. When Fang learned the truth behind the Soulsong she left for the Dead Dunes and formed the bandit gang Monoculus to search for the holy clavis needed for the ritual.
After awakening from crystal stasis and becoming the savior, Lightning reunites with Vanille in the cathedral after ridding Luxerion from the Children of Etro fanatics and Vanille hears of her exploits in the city, requesting to see her. Vanille shows Lightning a large concentration of souls in God's Sanctum below the cathedral, and Lumina explains the Soulsong is the Order's way of enabling Vanille to call the souls to her and save them from their suffering in the Chaos by offering them oblivion. Though Lightning doubts Vanille could handle the millions of souls, Vanille says she has no other choice, and Lumina says that if the concentration of souls ever escaped it would destroy the world and the future both.
In the Dead Dunes, Lightning reunites with Fang in search of the clavis. Upon finding it in the Temple Ruins Fang reveals her intention of destroying it to save Vanille. Lumina leads the Order's Secutors to the ruins and they claim the clavis. Lightning convinces Fang Vanille would listen to her and the two decide meet up in Luxerion on the final day when the ritual is to be performed. At the end of the eleventh day, Lightning is told the truth about the Soulsong by the dead who use Cid Raines as their medium. He explains Vanille has the power to save them all and help them be reborn.
On the final day Lumina reveals to Lightning the Soulsong's true nature: The Order seeks to wipe out the dead and erase their existence from the living's memories, purifying them of their burdened pasts so they can be reborn in the new world with a clean slate and live in bliss as per Bhunivelze's vision. The Order has kept the truth from Vanille and tricked her into believing the Soulsong will save the dead from their suffering. Lightning heads to the cathedral to stop the ritual and aided by Noel and Fang makes her way to God's Sanctum as Vanille begins the Soulsong.
Lightning convinces Vanille to listen to the dead and Vanille realizes the dead do not seek oblivion but carry on the hope of being reborn. As she chooses to stop the ritual and free the dead, the Order's high priestess uses a blast of energy from the clavis to strike Vanille down, symbolically stripping her of her sainthood and casting her out of God's favor. As the clavis drains Vanille's life it shines with a radiant light powerful enough to destroy the souls, but Snow arrives and uses Fang's spear to destroy the clavis, ending the ritual and killing the priestess. Fang and Vanille work together to guide the souls of the dead into the Ark so they can be reborn in the new world.
"The Soulsong" is the third track on the fourth disc of the Lightning Returns: Final Fantasy XIII: Original Soundtrack, and plays during the ritual.