As you explored the normal dungeons in the game, you may have come across some large vault-like doors at the end of the dungeons that would remain locked no matter what you did. The key to opening these doors lies in the fact that you have to finish all of the normal dungeons in the game first, with the exception of Pitioss Ruins, as that doesn't count. So, that means you will have to finish the likes of Balouve, Crestholm and Costlemark. Once you have done this, then travel to Meldacio Hunter HQ and speak with the older lady sitting on the chair, whose name is Ezma.
She will recognize your accomplishments and tell you about the eight great seals she was tasked with guarding. After her little story, she will hand over the Sealbreaker's Key and place a quest in your log, A Menace Beneath Lucis. This is an overarching sidequest that encompasses eight other sidequests, each related to one of the Menace Dungeons in the game. These Menace Dungeons are no joke and are the hardest ones in the game, ranging from level 55 and all the way up to level 99, with the latter being particularly nasty for another reason.
Preparing for Menace Dungeons
Naturally, you want to be strong enough to face down the trials that are the Menace Dungeons. As such, it might be a good idea to get your levels to the 90s, perferably level 99 for all members of your party. Of course, you don't need to do this right away, as you will likely gain a lot of experience going through the lesser Menace Dungeons, so there is some leeway here. The second thing you will want is a health supply of curatives, so make sure you get as many of the ones you can purchase from a shop and try and gather the ones you can't from Treasure Spots/Hunts.
For accessories, you will definitely want four Ribbons, as this will make you avoid having to concentrate on any nasty ailments that may be cast your way. One is gotten from finishing up the Sania quest line, while another is acquired for finishing the Dead General Strikes Down the King hunt in Lestallum (level 99). The other two will require you to procure 80 Oracle Ascension Coins and trade them in to the one NPC in the cafe in Altissia (40 coins for each Ribbon). You will be able to get a lot of Oracle Ascension Coins in Pitioss Ruins, should you be brave enough to explore.
Note: Due to the lengthy nature of the dungeons, full walkthroughs will not be given. Instead, the enemies that can be encountered and notable items will be listed.
|Keycatrich (level 55)||Ereshkigal, Galvanade, Black Flan, Bavarois, Marshmallow, Hecteyes, **Lakhmu Flan**||Dominator, Hyper Magnum, Wizard Shield|
|Glacial Grotto (level 65)||Yojimbo, Cryonade, Skeleton, Elder Coeurl, Royalisk, Kingatrice, **Kengo**||Vigilantes, Executioner, Black Belt|
|Fociaugh Hollow (level 65)||Yojimbo, Ziggurat, Ganymede, Skeleton, Aramusha, **Chandravarma**||Enhancer, Organyx, The Grand Chamberlain, Lavender Oil|
|Daurell Caverns (level 72)||Voretooth, Shieldshears, Malbadoom, Custard, Sir Tonberry, Coeurl, Dolce, Iseultalon, **Mahanaga**||Duel Code, Precision Lance, Field Medicine|
|Balouve Mines (level 78)||Voretooth, Hobgoblin, Killer Wasp, Imp, Mindflayer, **Psychomancer**||Apocalypse, Death Penalty, Emperor's Anklet|
|Steyliff Grove (level 86)||Havocfang, Imp, Psychomancer, Ronin, Matcha Mousse, Tonberry, Regaltrice, Killer Wasp, Royalisk, Kingatrice, Wyvern, Dolce, Mindflayer, Custard, Master Tonberry, Griffon, **Mictlantecihuatl**||Balmung, Flayer Lance, Aegis Shield, Robe of the Lord, Celestriad, Anklet of the Gods|
|Crestholm Channels (level 92)||Coraldevil, Killer Wasp, Hvitrormr, Seadevil, Gaiatoad, Phalaris, Havocfang, Malbodoom, **Manxom**||Blue Diamond Bracelet, Mighty Guard, Tarot Card, Anklet of the Gods|
|Costlemark Tower (level 99)||Havocfang, Hobgoblin, Mindflayer, Goblin, Malbodoom, Jormungand, Iron Giant, Braindrainer, Ultu, Elder Coeurl, Serpentess, **Bilrost**||Soul of Thamasa, Dark Matter Bracelet, Applied Sorcery, Hypno Crown|
Something you will notice as you begin exploring the Menace Dungeons is that you will come across rooms that have multiple exits. The thing is that only one of these exits will lead you further into the dungeons, while the others will lead to items. The only problem is that when you go on the correct path, a gate will block your way back to the previous room. This means that if you choose the path that goes deeper into the dungeon, you won't be able to go back and will have to redo the dungeon to nab those treasures. One little tip that you can utilize is that if you can target the enemies in the room from the hallway, then those paths will lead to treasure, while not being able to target them will lead you deeper into the dungeon.
There are two noteworthy exceptions to the Menace Dungeons, which are Steyliff Grove and Costlemark Tower. The former is just very long, consisting of "100 floors" and there will be camps on every 10th floor. You'll be around halfway done when you start encountering Kingatrices, and will be nearing the end when you are fighting Master Tonberries. The other dungeon mentioned, Costlemark Tower, is the hardest one in the game, simply due to the fact that you cannot use items while you explore it.
This is especially troublesome, as there are three very hard rooms inside. You can counter the No Item rule by using Healcast spells, as well as The Grand Chamberlain for Ignis (Fociaugh Hollow menace dungeon), since the latter bypasses the ability to use items. Also, it's a good idea to grind for some AP to get the First Aid abilities for your party members. The notorious rooms inside Costlemark are on floor 34, where you will fight five Iron Giants, and those with Elder Coeurls. Ribbons and the food that nullifies elemental attacks will make the Coeurls a joke. Having Limit Break magic (Freeze) will make the Iron Giants more manageable.