The Miasmal Citadel is an optional dungeon in MARDEK. It houses the first superboss of Chapter 3, Annihilator:Karnos. To access the Miasmal Citadel, you have to defeat the Bandits that Muriance sends after you when you give him the ★ Runestone Fragment. They drop a ★ Bandit Key which can then be used to open the warehouse in the Aeropolis slums. The shopkeeper here sells the ★ Eldritch Key which unlocks a door in the antechamber of the Dark Temple where Gope is. The door will lead to a separate part of the Sandflow Caves, which leads to the entrance of the Miasmal Citadel. You will have to put all the Miasmal Chalices, obtained from the Dark, Water, Fire, and Earth Temples on the vacant pedestals here. The portal to the "main" part of the Citadel will then activate. There is also another 2