This quest is started when you find the Unknown Tunnels. You must fight to the second level and either (a) rescue 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. There is no exit from the second level 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 and Marauder classes. Siding with the assassin lord is also a prerequisite for the Tricks of the Trade prodigy. As for the merchant... tough luck. Additionally later on in the game you will be able to reroll egos on your items. This is done in the strange portal after receiving a message from the assasssin's lord.
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. This also allows you to buy very powerful random artifacts in the late game.