The holy clavis isn't a key to anything. It's more like a coffin. It'll draw in the souls of the dead, to bring them the peace they deserve.
The holy clavis is an ancient artifact in Lightning Returns: Final Fantasy XIII essential to the performance of the Soulsong. It is desired by both the Order of Salvation and Oerba Yun Fang, who has traveled to the Dead Dunes where the artifact is said to be found, and founded the bandit gang Monoculus to help in her search.
This legendary relic is said to have been made by the gods themselves. It is used in rites to summon the souls of the dead.
It is thought that the clavis lies somewhere in the ruins of the Dead Dunes, and the bandit gang Monoculus have had their sights set on it for many years. The Order also shows a great deal of interest in the treasure, suggesting that it is no mere fairy tale, but a credible artifact from the past.
Both groups have searched the desert for the clavis, but have been stymied by powerful seals that keep them out of certain ruins.
Thirteen years before Lightning awakens from her crystal sleep, Oerba Yun Fang and Oerba Dia Vanille awoke from theirs in the city of Luxerion and were taken in by the Order of Salvation. When the Order discovered Vanille's ability to hear the voices of the dead they became interested in her and placed her under their protection in the Luxerion Cathedral as their 'saint.' The Order came to seek the holy clavis, an ancient relic with the power to summon the souls of the dead. Fang learned the Soulsong will kill Vanille if she carries out the ritual, and ventured off to the Dead Dunes to find and destroy the clavis.
When Lightning arrives to the Dead Dunes she is granted an audience with the boss of Monoculus, Fang, after performing a task for the gang. Fang explains she is seeking the holy clavis and after witnessing Lightning's powers as the savior, believes the relic is finally within her grasp. Fang and Lightning team up to find the clavis before the Order and depart for the Temple Ruins.
After searching the ruins, Fang and Lightning unlock the sealed chamber containing the clavis. When the duo come before the legendary artifact, Fang comments on a strange force pushing down on her chest. From the Ark, Hope Estheim tells Lightning the clavis was made either by humans with divine help or by the gods themselves. He alerts her to the approach of the Order's Secutors. Fang attempts to destroy the clavis, but Lightning stops her, only to learn what will become of Vanille if she uses the device. The Secutors arrive and Fang attempts to destroy the clavis against Lightning's wishes. Thanking the two for their efforts on behalf of "the sacred one," the lead Secutor summons a monster for Lightning and Fang to battle as a distraction while his men get the clavis.
When the monster is defeated Lightning and Fang discover the Secutors have taken the holy clavis. Fang reveals the clavis's true nature as a tool for destroying the souls of the dead, as well as the reason why she desired it in the first place: to keep it from Vanille. Fang states she teamed up with Lightning to ensure she did not hand the clavis over to the Order and Lumina appears, revealing she tipped off the Secutors regarding the clavis's location. Lumina marvels the holy clavis will destroy every soul trapped in the Chaos, including Serah's. Lightning demands to know when the Soulsong will take place, and Lumina tells her it will be on Nova Chrysalia's final day.
On the final day Lightning travels to Luxerion to stop the Soulsong and save Vanille and the souls of the dead. She teams up with Fang and when the two reach God's Sanctum they find Vanille has begun the ritual. As the souls are summoned to the clavis Lightning and Fang tell Vanille the truth about the Order and the Soulsong. Lightning tells her she has the power to lead the souls of the dead to the Ark to be reborn in the new world. Vanille agrees to stop the Soulsong, angering the Order's high priestess, and a blast of energy from the clavis strikes Vanille down. As the clavis drains Vanille's life it shines with a radiant light with the power to destroy the souls, but Snow Villiers arrives and uses Fang's Bladed Lance to destroy the relic and obliterate the high priestess.