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.

How to unlock the Dark Citadel

There are three primary things you need to do to unlock the new Dark Citadel dungeon in Diablo 4:

• Finish the Vessel of Hatred story quest
• Reach Torment 1 world tier
• Complete the Crater of Lost Souls quest

That first part is simple enough, but the second part is much more complicated. After you finish the main quest, you'll get another from Ormus in Kurast called Deeds of a Champion, which is essentially a checklist of activities you have to complete to unlock Torment difficulty. 

In order, these are:

1. Complete quests “A Nameless Mystery,” “Slayer's Retribution,” and “A Feather On the Scale” (these are the mercenary recruitment quests you find in The Den).

2. Conquer the Stronghold Kichuk. Complete the questline “Tenets of Depravity”. Complete three dungeons in Nahantu.

3. Complete quest “The Kurast Undercity” then quest “Undercity: Wail of the Forgotten” (these are the quests that unlock the Undercity activity).

4. Gain 10 Grim Favours in Nahantu (complete whispers in Nahantu and claim a Tree of Whispers reward cache).

5. Travel to a Helltide region. Open five Tortured Gifts. Slay a Blood Maiden.

6. Reach level 60 and unlock The Pit of the Artificers.

7. Complete a tier 20 Pit of the Artificers run to unlock Torment world tier

8. Change to Torment 1 world tier and complete the Crater of Lost Souls quest to unlock the Dark Citadel.

It's quite a long process, but as soon as you get to the Torment world tier, you'll get the Crater of Lost Souls quest that unlocks the Dark Citadel.