The Unknown Tunnels are hidden and do not appear on the map. Rather, they are keyed by a random encounter on the world map (in Maj'Eyal). You must be at least level 6 for this encounter to occur. The encounter text is:
- You find a hidden trap door, and hear cries for help from within...
Choosing to enter will put you on level 1 of the zone.
The top level is a darkened maze of single-wide corridors, with no exit back to the surface, and only a single (randomly placed) exit leading downward. The second level has a fixed layout with some rectangular rooms on a lighted, open level, with some NPCs (see below).
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 (needs verification for 1.0). 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, Shadowblade 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.
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.