The Promise.A brief flashback sees a young Phi and Lux saying their goodbyes as they separate. In the present, Phi continues to strangle Lux but she suddenly passes out and Hayes appears beside them. Back in the ruins, Lishe and Krulcifer defeat the remaining Abyss. Airi tells them that they had discovered the entrance to the deepest parts of the ruins but they lack a key to enter. Back in the old building, Lux confronts Hayes but she warns him that if she dies, Phi dies too. Hayes explains that she planted a seed of Ragnarok inside Phi and that she is slowly turning into an Abyss as her body is being eaten from the inside. Hayes also reveals that Phi had been resisting her commands which explains her fever and headaches. She then offers Lux a deal in which she will command the Ragnarok to save Phi who will die in a few days in her current state but in exchange, Lux must open the entrance to the deepest part of the ruins using Krulcifer as they key but he is given a time limit of two days to accomplish this. Later, Lux meets with Relie who reveals that Phi supposedly died due to the inhumane experiments conducted on her five years ago but she awoke stronger than before but at a terrible price. Relie also reveals that her determination to explore the deepest areas of the ruins is because she hopes to find something that can cure Phi. The next day, Lux reminisces about his past again while his Bahamut undergoes maintenance. To his and Airi's shock, they are suddenly contacted by their Fugil who explains that the only way to save Phi is to destroy the Ragnarok controlling her and Hayes' horn. Before returning to the ruins, Lux visits Phi who reveals that she knows the truth of her condition but states she is satisfied as both Lux and Relie have grown strong over the years. The group opens the entrance to ruins' deepest area with Krulcifer's help and La Krushe, whose memory has recovered, attacks them as Hayes appears with Phi under her control. Airi, who also possesses a horn, allows Phi to regain control of herself and breaks Hayes' horn in the process but the Ragnarok is revived. Despite their best efforts, the group is overwhelmed until Lux unleashes Bahamut's full power and defeats the Ragnarok. Fugil convinces Hayes to retreat as the ruins begin to collapse and La Krushe loses consciousness. As Lux recovers, he is assured by Phi that she can resist the horn and thanks him for saving her.