Ground Level (not shown) You'll find the entrance to T'sen-Ang through the tree canopy to the northeast. However, the two guards next to the staircase there won't let you go up unless you have a Molochean Hand amulet. If you don't have one, the easiest place to find one is on Bingham Schulefest at the Caladon cemetery. You can also just wander around the world map until assassins attack you, and then pick up an amulet from them. Finally, if you're a good-aligned character, you can simply attack the guards to go up (you'll probably end up fighting everybody at T'sen-Ang anyway), but be aware that there are about a half dozen guards in the area. Most are hidden behind trees and are difficult to spot.
When you leave T'sen-Ang, if you have Virgil or Vollinger in your party, it's possible they'll take the opportunity to leave you. If the dark elves on the ground don't attack you then one of them will talk to you and leave automatically, but if the dark elves do attack you then they will only leave if you talk to them prior to exiting the area (you might need to climb up and down the stairs to trigger the conversation).
If Virgil leaves, you'll find him in the basement of the Sobbing Onion in Caladon (the trap door is in the room with Mr. Razzia). You'll probably have to resurrect him (you should find a scroll of resurrect in a chest there), but you'll finally learn Virgil's history, and you'll finally meet Brother Joachim. In early versions of the game, the episode caused Virgil to undergo a metamorphosis (among other things he lost all of his equipment and changed to level 25), but in the most recent version he remains exactly the same character.
If Vollinger leaves, he'll mark Vollinger's Meeting Place on your world map, and he'll ask you to meet him there alone. So pick a nearby location (like the Pit of Fires), tell your companions to wait, and then head for the meeting place. In the basement of the house there, Vollinger will reveal that he's working for the Molochean Hand, and he'll attempt to kill you.
1 - Z'an Al'urin 
Z'an is an evil spellcaster (earth, force, and temporal). If you tell her your name, she might agree to join you.
2 - T'val N'or
3 - Half-ogre Slaves 
If you talk to them you'll learn that the dark elf wizard T'val N'or (#2) holds them in thrall. But if you can kill T'val, then the half-ogres will go free and attack the dark elves for you. You'll get a fate point for freeing the half-ogres, but you should wait until you do everything else you want to do in T'sen-Ang first.
4 - M'in Gorad's Residence 
If you're persuasive, you can do a couple things when you talk to M'in Gorad. You can claim the bounty for killing the survivor of the zeppelin crash (that is, you). It's worth 2000 gold. You can also trick M'in Gorad into revealing that the Black Mountain Clan was banished to the Void to help build a technological device to weaken the wards on the Ring of Brodgar to pave the way for the return of Arronax. That's worth 7600 experience.
If you're not persuasive, then you can kill M'in Gorad and learn about the BMC dwarves from a note on her corpse. The dark elves outside the residence won't realize anything untoward has happened.
If you're evil, then you can offer to join up with the dark elves. M'in Gorad won't quite trust you, and she'll give you a quest to slaughter the town of Stillwater. That shouldn't be too difficult, and it will even earn you a fate point. Plus, you'll earn about 16,000 experience when you return to M'in Gorad. Then she'll tell you about the BMC, and ask that you meet up with high priest K'an Hua at the First Panarii Temple.
Note: If you join up with the dark elves then Qintarra will turn against you. You also won't be able to go to Thanatos or complete any quests in Tulla (including the magic college mastery quests).
After learning the fate of the BMC and leaving M'in Gorad's residence, you'll run into Gideon Laier, the First Blade of the Molochean Hand. He'll recognize who you are. If you know something of the history of the Molochean Hand, you might be able to convince Laier to leave you alone by telling him that he's essentially working for Arronax. However, since Gideon is wearing some nice equipment, you might want to let him attack you so you can get it. None of the dark elves will come to his aid. Oddly, the conversation with Laier will go the same whether you've joined up with the dark elves or not.
If you have read Joachim's book and learned about First Assassin Trellian, then you can confront Gideon about the Hand repeating the mistakes of the past ("The Hand hasn't always been this way...") . He will promise to call off the attacks on you, and if Vollinger is in your party, he will out him as a Hand assassin and offer him to you as a permanent companion, to make up for the Hand's attempts on your life.
A. Staircase down to the surface.