To unlock the ??? Cave, you first have to kill Cloud of Darkness which you already did if you follow this walkthrough. Now reload the game with the save after end credit (save slot with the 3 stars), this will bring you back to the Crystal Tower.
Second, you have to master at least one job by reaching level 99, then find the Legendary Smith and talk to her. See Mastery Items and Cards (Optional) for more detail.
Once you completed the second requirement above, board The Invincible and talk to the MogNet on the left as Luneth, you should receive a new mail from the 4 Old Man, read it. Next, change the top character to Arc and talk to the MogNet again, read any new letters from king Alus until you see "Saronian Legend" which tell you about a legend of a falling star into the sea.
That's it, now the ??? cave is appeared at the south sea between Floating Continent and Doga's continent, see map below:
The ??? Cave has only 3 monsters:
- Yellow Dragon
- Green Dragon
- Red Dragon
They all drop Elixir and Onion equipment. The cave is pretty short, only a few steps toward Iron Giant.
Boss: Iron Giant
This boss can inflict various status ailments with his attacks, so having Ribbons or Onion equipment will make the fight an easy mode. First, cast Haste and Protect on everyone, then you can focus on offensive mode by having 3 members attacking and 1 healing.
If you can't beat this boss, simply because your level is too low.
You'll get Metal Card after you win the battle.
