This quest is started when you find the Unknown Tunnels. You must fight to the second level and either (a) rescue Urthol, the Merchant, or (b) side with the Assassin Lord.
On level 2, you will find a merchant (nonhostile), the Assassin Lord, and assorted rogue-type henchmen. For most characters, the assassin lord and his men will also be nonhostile, and you may talk to the assassin lord. For Celestial characters in the Sunwall faction, the rogues will be hostile from the start. The Assassin Lord will take no action at first, even if he is hostile. There is no exit from the second level (apart from using the Rod of Recall) until you have spoken with, or killed, the assassin lord.
If you talk to the Assassin Lord, you may choose to side with him. This permanently unlocks the Poisons tree for Rogue, Skirmisher, Marauder, and Archer classes. Siding with the assassin lord is also a prerequisite for the Tricks of the Trade prodigy. As for the merchant... tough luck. Additionally, after returning from the Far East, if you have the Forbidden Cults expansion enabled, you will be able to reroll egos on your items. This is done with the Font of Sacrifice, which becomes available after inputting a code into the Occult Egress. You will receive a message with the code from the Assassin Lord when you first enter Last Hope after returning from the East.
If you talk to the Assassin Lord and choose to attack him, or if you attack him without talking to him, he will throw a smoke bomb and disappear (teleporting randomly within radius 10), activate his sustained talents, and start attacking the player. If this was triggered by hitting him, he will take no damage from that first hit.
If you kill the Assassin Lord, and do not kill the merchant, then the merchant will give you 8 gold (this used to be worth a lot more in earlier versions), and will open up his store in Last Hope. Apart from selling items, he also sells the to characters who know Trapping for 3000 gold, and he sells the to characters who know Poisons, and have 40 Cunning and Level 25, for 1500 gold. After returning from the Far East, you are able to order very powerful random artifacts via the merchant for 4000 gold each. When ordering one of these artifacts, you can choose which material tier 5 item to use as a base (i.e. you can choose what equipment slot you buy an item for, and choose between different types of item for that slot, but you don't see exactly what the item will be before you buy it). You can name the item made this way.