Fortress of the Black Mist
Fortress of the Black Mist was built as a filter, not a source, and every stone of it was laid by people who understood the difference. Something under the island produces the mist. The fortress exists to slow the rate at which it reaches the surface, and it has been failing at that job by small increments for a very long time while remaining fully staffed.
The garrison is what makes the place grim. Mist wraiths and shadow mages at the outer works are what the mist does to a person given long enough. The void sorcerers and arcanists deeper in chose this, took the posting, and are still filing the reports the posting requires. They are not defending the fortress from you. They are defending a failing machine from being opened, and you are the thing trying to open it.
Encounters
Tier 1
Mist Wraith
Tier 2
Shadow Mage
Tier 3
Void Sorcerer
Tier 4
Dark Arcanist
Tier 5
Morven (Mini-Boss)
Tier 6
Nether Warlock
Tier 7
Void Conjurer
Tier 8
Shadow Lich
Tier 9
Dark Archmage
Tier 10
Necromancer Vel-Khazir (Final Boss)
The final boss
Necromancer Vel-Khazir is the fortress's current custodian and the reason it is still closed at all. He has kept a mechanism running past the death of everyone who knew how it worked, using the only labour available to him, which is the dead. He is not guarding a secret. He is holding a lid, and he will not step aside to explain that to you.
Before you enter
The recommended level is 30. Treat it as a starting point: farm earlier tiers, upgrade gear, and adjust your pet when a boss stops you.
Difficulty inside a dungeon comes from one number that climbs with the tier, and the step between tiers is a large fraction of the gap between dungeons. So a wall at tier 7 here is not a signal to grind this dungeon harder - it usually means the opening tiers of the next dungeon are a better use of the hour, and you come back with the gear they dropped.