First, you must complete the following storyline boss battle in order to gain access to the area:
Once that is done you have 2 choices:
*** Spoiler - click to reveal ***[1] Continue with the story by going though the glowing green portal.
[2] Move back from the boss battle area, go down the steps on the left/right and go through the entrances beyond the tents.
Go with [2] and you'll gain access to the Karba Prison.
Before you can attempt this Achievement you will need to find a Mystic Key. This is located in the Nadir overworld area, just outside of the Antivirum Cavern. You will need use Taika to dig at a specific spot, but this can only be done if you have acquired a certain Mystic Treasure Map and used Ive's ability to reveal the co-ordinates. More info on that here:
https://www.xboxachievements.com/game/earthlock/achievement/... The prison is made up of 2 floors, each with a number of gated entrances.
Every time you open a gate it will trigger a boss battle.
These boss battles usually feature stronger versions of enemies you fought earlier in the game.
Every time you defeat one of these bosses you will find a new Mystic Key inside the room you just opened. This key unlocks the next gate.
The first gate is on the bottom floor, far left. You then move from left to right opening each gate in succession.
Once the bottom floor is done you repeat the sequence on the top floor, with the central gate being saved for last.
Once you open this central gate you will unlock the Achievement. This gate will not trigger a boss battle, however you can choose to fight enemies once you enter.
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The enemies you fight through these battles can be very tough, so a well-developed party is recommended.
Also, you can leave the Karba Prison at any time without losing progress, so take advantage of this to Save at the nearby Onurasi Statue. I would also recommend warping back and forth between Karba and Plumpet Island in order to fully heal your party.