Where The Clavis Lies, or Lightning Returns: Final Fantasy XIII main quest 4-3, takes Lightning and Fang inside the Temple Ruins, looking for the Clavis, but entering the Temple is not an easy job.
The third main-quest in Dead Dunes, as well as the fourth, can be accessible if you have all key items required in the ruins, allowing you to avoid a possible return to the surface, but if you don’t have them you will waste time travelling to the surface and then back inside the Temple.
Where The Clavis Lies is unlocked after you meet Fang (Aspiring Bandit main-quest) in Ruffian; and after she agrees to join your party.
Along with her, you must open the gate to the Temple Ruins, in the north-easternmost corner of Giant’s Sandbox; so you can enter Chamber of Floods (Sacred Gate) .
Description: “Fang has been using her position as head of the bandit gang to search for the clavis, but she does not disclose why she wants an ancient relic that gathers the souls of the dead. Meanwhile the Order is redoubling their efforts to find the clavis. If Lightning and Fang are to reach it first, they must head quickly to the ruins in which it is buried. The entrance stands near the Giant’s Hand.”
Location: Dead Dunes
Requirements: Complete main-quest 4-2: Aspiring Bandit
From the start, you should be aware that the Temple Ruins is a difficult dungeon, with multiple floors, and a large number of enemies that will constantly attack you.
Buy potions to restore Lightning’s health, remove status ailments, and don’t forget to upgrade her abilities. While the enemies inside the Temple are common beasts, you will also encounter Earth Eaters.
Furthermore, once you enter the temple, you can explore as many chambers as you want; however; the purpose of the walkthrough is to guide you through the maze up to the point where the next quest (Murals of the Gods) begins. In other words, the walkthrough shows the fastest path you must follow to end the current quest, without focusing on exploring and gathering items, unless they are key items required to complete the quest.
When you are ready to enter the dungeon, head NE from Temple Ruins North Gate, followed by Fang. When you get there, look for the exclamation mark near the entrance, and then talk to the guard to open the gate.
After the gate opens, follow the Path of Ashes (there is only one tunnel available), and you will enter Chamber of Dusk: Upper Level. Activate the Red Cactuar Statue, continue south, and then head right (west on the minimap). Next, move north and jump through the hole to enter Chamber of Dusk: Lower Level, where you will learn about the Timed Doors. Take the southern door, and turn right in the next hall to reach Path of Ruin.
Follow the long tunnel smashing all skeleton statues and gathering the Yellowed Skulls they drop. When you enter the Chamber of Plenilune: Upper Level, continue to move south, then west, and north, to reach another hole and a ladder. Drop down one level, and you will find yourself in Chamber of Plenilune: Lower Level.
Move south, away from the ladder, and cross the large hall. When you reach the other side, turn right to see a smaller chamber and a gate with red symbols. Notice the red cube on the left side, and activate it to open the door that leads to the Path of Vicissitude. Follow the next tunnel and you will enter the Chamber of Flame.
Continue to move west (left on the minimap), and cross the Sentry Row passage to enter Chamber of Floods (Sacred Gate). Activate the Red Cactuar Statue in Chamber of Floods (Sacred Gate) because you will need it later.
After you unlock the Temple Ruins: Sacred Gate warp point, check the large door on the other side of the hall, as Fang asks you.
Watch the cutscene that ends the quest, and get ready to return to the surface. At this point, Where the Clavis Lies ends, and the next main quest named Murals of the Gods begins.