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 18 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). You will find a merchant (nonhostile), the Assassin Lord, and assorted rogue-type henchmen. This is the body of the Trapped! quest.
REWARD: Solving said quest by rescuing the merchant will give you access to his store (selling 'all sorts'), in Last Hope. Later in the game, he will also sell very powerful random artifacts for 4000 gold each. If you solve the quest by siding with the Assassin Lord instead, then later in the game, the Font of Sacrifice will become available, which lets you pay gold to reroll the egos on your items. There are also other, smaller rewards for both options. See the quest page for more details about the rewards.
Of note is the presence of summoning traps, which when stepped on surrounds you with thieves. This can pose a significant danger for an unaware character, particularly if lightly armored or susceptible to sudden perforation.