It's part of the Rebellion in Westonia / General Kaidun story arc, particularly the Kaidun story quest line.
Citadel has very complex layout. Entrance part has six levels and route to Catacombs part, which has three levels. Necromancer Tomb story dungeon in found on the lowest level.
First part of Citadel mainly comprises the long stairs that lead all the way up to the top level. Staircase has numerous side corridors and chambers on every level.
Lowest level is mostly safe and has federation camp: Gloomhaunt Sentry Post
Hall of Maidens
Many rogue vampire maidens work with the remaining Witches of Vadoma to conduct crude magic. The halls are flooding with gore and the walls are spattered in blood.
Ages ago Amani warriors stormed Citadel of Torment with Kaidun himself. By the time they realized the halls were filled with the worst nightmares, it was too late to retreat. Thulsa sent the Cursed Vampir Gentlemen and Infernal Jesters to attack them. Most of them went completely mad, and some changed into hideous creatures. Some of his comrades were turned into Eldritch Defenders who now patrol the main stairs.
Hall of Masters
The vampire lords and maidens have taken over the east wing with their undead brutes causing chaos elsewhere. Lower level was for ladies, Vampire gentlemen habit the Hall of Masters.
Corrupt and sadistic Curser Jesters make this their home. They constantly cause chaos amongst the inhabitants and take joy in ending their lives. The disease from the Vertigo Reach floods in through cracks in the ceilings and cause madness to spread through the halls. There is also a room with Toxic Jesters you have to find.
Strange shapes have been seen in the clouds around the terraces and edges. Monsters of pure mist wander the halls. A strange fog spreads an odd disease, similar to the Mad Blood disease, that is adding immensely to the torment of those inside.
They could have put some secret or easter egg or world boss here. They didn't. It's just Malevolent Wraiths.
Within this highest portion of the citadel, horrible torture is committed. The few who survived these acts soon found themselves jumping off the terraces. Ending any source of suffering up here would be the most honorable and respectful thing someone could do for them now.
- NPC: Helehr, former Aman, now Sihran
If previous levels had Vampirs and Jester, now you have both in same room. There is one bigger room in east side. If there was something, it's long been removed. There is a statue of the same guy as main stairs have. It doesn't look like Thulsa 'thou.
South of the main stairs is why you come here. Last room on west has broken roof and floor and you can jump few floors down into the Catacombs.
Sub levels aka the Catacombs
If you entered this area by climbing up to Anguish Heights level of Citadel of Torment, then climbed down the broken floors, you end up in Infiltrators Refuge camp. Luckily there's teleport master who can move you between the camps.
This level is totally uninteresting and has about eight copypasted rooms with same odd plant thingie and two Lokian Blood Guard Lancers. Main point is to find stairs down. Well, there are also eight stairs down, so it's not too hard, and you end up in some part of Catacomb of Misery.
Catacomb of Misery
Very large level that is mostly linear. Stairs up (entrace to this level) are on left side of map, stairs down to next level are on right side of map. West part is named "Distortion Prison". All enemies in Distortion Prison are Lokians.
Federation Sanguine Hideout camp is located on north-eastern part, almost along the only route from left side of map to right.
Lowest level of the Catacombs. There are stairs where you can descend to this level. Area is quite large, but there are only few types of rooms that are copy-pasted few times, in addition to few big halls. You will get lost without looking the map.
Your aim it to navigate through Rotting Prisoners, Melkonari Magisters, Ebon Inquisitors and disc reapers (you thought you never see these guys again!) to the southmost middle chamber to the Necromancer Tomb teleporter. Or just use vanguard menu or teleport scroll to get there.
There are lots of additional quests from the camp on upper level.