The Holy Clavis main-quest feature in Lightning Returns: Final Fantasy XIII, is the final quest in the area known as Dead Dunes, and as most main-quests it can be completed only if you manage to defeat the boss featured in the area.
In Dead Dunes, this boss can be Grendel, or Parandus if you open the Sacred Gate after the seventh day. No matter who you will fight, the rewards are the same, as well as most of the tactics used by these two bosses.
Throughout the following guide, we will assume that you have focused on completing as many quests as possible, thus you delayed your arrival in Dead Dunes, and that you fight Parandus.
The Holy Clavis begins after you get the Crux Base (The Murals of the Gods quest), and after you open the Sacred Gate.
Quest Details
Description: “Lightning and Fang have seen the three murals. They have three pieces of the crus, and now they can open the Sacred Gate. Soon, the holy clavis will be in their hands. But the Order is closing in on the treasure as well. The pair will have to hurry to the chamber, if they are to claim the prize and keep it out of the Order’s hands.”
Location: Dead Dunes
Difficulty: N/A
Requirements: Complete main-quest 4-4: Murals of the Gods
Failable: No
Rewards: Maximum HP + 540, Strength + 28, Magic + 56, Maximum ATB + 10, Recovery Item Capacity + 1, Preta Hood
Quest Walkthrough
Note: Before engaging Parandus, you should return to Ruffian and restock on potions, boost Lightning’s abilities, and make all necessary preparations. Parandus is a powerful enemy but it is weak to poison. Wind-based attacks are also required to be able to stagger the boss.
When you are ready to begin, head through the gate and watch the cutscene that precedes the fight.
As the battle begins, Parandus will start casting Breath of Rancor which is a physical attack you can guard, but it also debuffs Lightning with poison.
Try to remove it with Esunada because it will slowly drain her health. Next, start casting wind-attacks to stagger Parandus. Aerora is the best choice because it also penetrates the Causality Barrier, which protects the boss from debuffs.
While the Causality Barrier is up, your opponent is immune to all debuffs; and all attacks, except those based on wind, are halved. Continue to cast Aerora and if you are hit by Disaster, immediately cast Esunada, as you will be debuffed with Debrave, Defaith, Deprotect, Deshell, and Curse, making Lightning extremely vulnerable.
After you stagger Parandus/Grendel, debuff the boss first, casting Deshell, Deprotect, Slow and even Defaith, then switch to physical or magic attacks because your opponent is vulnerable to all of them.
Note: Curse is the only debuff that won’t stick.
Poisoning Parandus is a good idea if you want to inflict damage over time. When attacked, pay close attention and if you see your opponent getting ready to cast Shadow Clan, make sure you guard it because it is powerful 8-hit magic attack.
When the boss recovers after the first stagger, start casing Aerora to stagger it again, followed by all debuffs mentioned above and physical or magic attacks. Once defeated, watch the cutscene and pick up the Preta Hood on the ground.
Approach the exit and Fang will leave the party. The moment you finish The Holy Clavis, all side-quests in Dead Dunes area become available.
To continue the main-story; however, you have to travel back to the Wildlands and start Father and Son.
