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Fang's Oath (突き立つ牙の誓い, Tsukitatsu Kiba no Chikai?, lit. The Piercing Fang's Oath) was a single-track Challenge Event in Final Fantasy Record Keeper, set in the Record Realm of Final Fantasy XIII. The event encompasses the events in the Hanging Edge, Palumpolum, and the Fifth Ark. The event's main character is Fang, with Lightning, Sazh, and Hope returning as recruitable characters. All four also received their Memory Crystals. Vanille also returns by means of a Bonus Quest. In the Bonus Battles, two of the main prizes were Lifesiphon and Lightning's Memory Crystal II. In the global release, this event ran from January 14 to January 24, 2016 (UTC).
Lightning successfully derails the Purge train out of Bodhum, with Sazh's help, triggering a massive response from PSICOM forces in the area. The security unit sends a Manasvin Warmech to subdue them, but it is defeated quickly. So begins the chase from the greater Sanctum as Lightning, Sazh, a boy named Hope, and resistance leader Snow stumble upon the remains of a fal'Cie sanctuary. Joining them is the mysterious Vanille, a girl whose origins are yet unknown. The four are selected as l'Cie by Anima, who gives them a Focus to complete. At the same time, they are labeled dangers to Cocoon society, fit to be destroyed.
Some time later, the l'Cie manage to infiltrate the city of Palumpolum, each with personal objectives. Snow and Hope are waylaid by an Ushumgal Subjugator before reaching the Estheim residence as the others continue to evade PSICOM around the city. They eventually meet with Hope's father to plan their next moves, but are cornered by a Havoc Skytank. Though a particularly long battle, the Skytank is also dispatched.
Fang, a l'Cie working with the Cavalry unit of the Sanctum, safely guides Lightning's party out of Palumpolum, but soon after, they find Vanille and Sazh captive aboard the Palamecia, and must escape again. Their escape vehicle crashes into ruins identified as the Fifth Ark, whereupon the party encounters Fang's former superior, Cid Raines, himself turned l'Cie by the Sanctum and set on obliterating the party. He too is cut down.
Meanwhile, Fang's shoulder begins to burn, releasing the eidolon Bahamut. Her trial is about to begin.