I'll save them all!Vanille begins the Soulsong
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.Lumina, describing the Soulsong
The god Bhunivelze proposed the ritual 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 considers rebirth in the new world an honor reserved for the chosen and dedicates itself to performing the Soulsong.
The Soulsong needs two things: a sacrificial 'saint' who can hear the dead's voices and summon them, and the holy clavis, a powerful artifact forged by the gods for the express purpose of gathering the dead and destroying them. The Order dubbed Vanille their saint, as she 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.Lightning to Fang
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' as their revered saint, confining Vanille to the cathedral. When Fang learned the ritual's truth, she tried to leave the Order and take Vanille with her, but Vanille chose to stay to prepare for the Soulsong and atone for the past. Fang 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 rids Luxerion of the fanatical Children of Etro. After hearing of Lightning's exploits in the city, Vanille requests to see her and shows Lightning a large concentration of souls in God's Sanctum underground. Lumina explains the Soulsong enables Vanille to save the souls 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 both the world and the future.
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 that Vanille would listen to her and the two decide to meet up in Luxerion on the final day when Vanille will perform the ritual. At the end of the 11th day, the dead use Cid Raines as their medium to tell Lightning the truth about the Soulsong and explain 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 to erase the living's memories of the dead. The Order 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.
After she and Fang tell her the Soulsong's true purpose, Lightning convinces Vanille to listen to the dead. Vanille realizes the dead don't seek oblivion but sustain the hope of being reborn. When 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. As the clavis drains Vanille's life, it shines with a radiant light powerful enough to destroy the souls. Snow arrives and uses Fang's spear to destroy the clavis, ending the ritual and killing the high priestess. Fang and Vanille guide the souls 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.
Behind the scenes
One can compare the Soulsong and Vanille's intended use as a sacrificial offering to ritual purification and Jesus's role as the Lamb of God, whose sacrificial death as a servant of God purged humanity's sins and purified them. As Bhunivelze's followers, the Order sought to use Vanille as a sacrificial lamb and have her die in a ritual to destroy the dead and 'purify' the living of their burdens in preparation for their rebirth in the new world.
When Vanille loses her headdress after the high priestess strikes her down, one can see the action as the priestess symbolically stripping Vanille of her sainthood as punishment for her defiance against God, or Bhunivelze casting Vanille out of his favor through the priestess.