Holy clavis

The Holy Clavis is an ancient artifact that appears in Lightning Returns: Final Fantasy XIII and is essential to the performance of the Soulsong. For this reason, it is desired by the Order of Salvation, as well as Oerba Yun Fang, who is determined to find and destroy the clavis before the Order can get its hands on it. All for the purpose of obtaining the clavis, Fang traveled to the Dead Dunes, where the artifact is said to be found, and founded the bandit gang Monoculus.

Datalog
''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.

Story
Thirteen years before Lightning awakens from her crystal sleep, Oerba Yun Fang and Oerba Dia Vanille awaken from their own in the city of Luxerion, under the care of the Order of Salvation. When the Order discovers Vanille's ability to hear the voices of the dead, it becomes "interested in her" and comes to seek the Holy Clavis, an ancient relic with the power to summon the souls of the dead. Fang learns the Soulsong will kill Vanille if she carries out the ritual, so she ventures off to the Dead Dunes in hopes of finding and destroying the clavis to save her dear friend's life.

When Lightning finally comes to the Dead Dunes herself and vanquishes the sandstorm known as "God's Wrath" for Adonis, a member of the bandit gang Monoculus, she is granted an audience with the man's boss, who turns out to be Fang. Fang reveals her desire for the Holy Clavis and her inability to find it thus far, but after witnessing Lightning's powers as the Savior, she believes the relic is finally within her grasp. Fang and Lightning team up to find the clavis before the Order does, and they depart for the ruins.

After much searching within the ruins, Fang and Lightning finally unlock the sealed chamber containing the Holy Clavis. When the duo finally comes before the legendary artifact, Fang comments on a strange force pushing down on her chest. From the Ark, Hope Estheim tells Lightning that the clavis was made either with divine help or by the gods themselves. He then 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 the girl uses the device. The Secutors finally arrive in the clavis's chamber, and Fang attempts to destroy the clavis again against Lightning's wishes. Thanking the two warriors for their valiant efforts on behalf of "the sacred one," the lead Secutor summons the Grendel/Parandus for Lightning and Fang to battle as a distraction while his men get the clavis.

When the Grendel/Parandus is successfully defeated, Lightning and Fang discover the Secutors were successful in their mission to obtain the Holy Clavis after all. 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, rather than to give it to her, as Lightning believed. Fang lastly states the only reason why she teamed up with Lightning in the first place was to make sure she did not hand the clavis over to the Order herself, and Lumina appears shortly after. The enigmatic girl reveals it was her who tipped the Secutors off regarding the clavis's location, angering Fang, and she also shows her interest in the Soulsong and how the Holy Clavis will destroy every soul trapped in the Chaos, including Serah's. Lightning demands to know when Vanille plans on using the clavis, 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 again, and when the two reach Vanille's location, they find that the girl has already begun the cursed ritual. As the souls are summoned to the Holy Clavis, Lightning and Fang tell Vanille the truth about the Order and the Soulsong. While the Order's high priestess demands that Vanille must finish what she started, Lightning tells her that she has the power to lead the souls of the dead to the Ark to be reborn in the new world. Vanille finally agrees to stop the Soulsong, angering the high priestess, and a blast of energy from the clavis symbolically casts Vanille out of God's favor. The clavis begins to shine with a radiant light that possesses the power to destroy the souls, but Snow Villiers arrives and uses Fang's Bladed Lance to destroy the divine relic once and for all.