To get to the second floor of the Dungeon of the Torturers, you must build a Torture Rack and interrogate a Torturer Scout on it. You'll find all the unique items needed to build the Torture Rack on the first floor of the Dungeon of the Torturers.
To catch a Torturer Scout, you will need a Lvl 3 Scroll of Control. Go to northern locations or the surface of dungeons, the Catacombs, or the Shrine. When you see a Torturer Scout, use a Scroll of Control on them and leave the location while the scroll is still active. Return to your castle and get information from the Torturer Scout, whom you will find on the Torture Rack. The information you get from the Torturer Scout will give you access to the second level of the dungeon. Within 96 hours of the second torture, the Torturer Scout will show you the shortest route to the final chest. Just follow the arrow on the dungeon minimap.

On the second floor of the dungeon, you will find traps, puzzles, and unique enemies. Apart from the final chest, the rooms contain additional treasures. The most attentive Exiles have the chance to discover new passages and even the Secret Room.
Inside the dungeon's chests, you will find new designs: the Blast Furnace Design, which will come in handy for making steel ingots, and the Iron Maiden Pannier Design. Equipping the Merchant Pannier gives you an additional quick slot, which you can use even if your character is wearing light armor. Other rare rewards that you can find on the second floor of the Dungeon of the Torturers include parts of the Gladiator Set, the Bear's Paw, the Executioner's Hook, and the War Dogs Axe.