The Dark Citadel is the newest endgame activity added in Diablo 4's Vessel of Hatred expansion, and Blizzard is definitely trying something new here. Unlike the pretty straightforward dungeons of the main game, this endgame dungeon has three separate wings containing bosses with special mechanics, and you can even get unique cosmetics and loot by completing it.
It is quite a long-winded process to unlock it, though. You'll have to both complete the Vessel of Hatred story and then a questline which essentially runs through every new mechanic and endgame activity. Here I've outlined the exact steps to unlocking the Dark Citadel so you can try it for yourself.
