Find the pieces of the old lithograph and face-off with Box Head.
This is a linear level but you need to be careful of traps that will kill you in one go. As you begin, go forward and snatch up any loot you can find. Near the stairs you’ll be attacked, so go for headshots to put them down quickly and torch the bodies. You can always back up to keep your distance from these enemies.
Near the three torches at the far end of the hallway you will be prompted to interact with them and be given your first Old Lithograph. You need to find three of these in total. There are some supplies in this room too, including the Catacomb Note. As you continue you find a room with a monument and slots for three Lithographs. Place your first one here.
There are doors to the left and the right here and another to the mirror and the save location. Use this throughout your exploration here, allowing you to level up and regenerate health as you go.
Go left and you’ll find a room with a bunch of cells and at least three enemies. Dismantle the traps and wait for the enemies to close in before shooting the explosives to take them out. Loot the cells and then climb down the ladder. There’s one enemy here which you’ll need to take out before turning the two valves to get rid of that pesky oxygen meter.
Climb down the next ladder and take out the enemies here while looting anything you can find. Crank the wheel to open the next area and you’ll be closed off from everything behind you, so make sure you take everything you want. Disable the trap on the ground, burn the enemies on the floor and head to the next crank, which again closes off the path you’ve just come from. Continue until you reach the stairs and you’ll find the second Old Lithograph and a route back to the monument. Place the Lithograph on the slot and save your game before going through the last door (the one on the right).
Take note of the trap here and how it works as you can use a few like this in this area. Disable the trip wire as you continue forward and you’ll see two wheels that look like they can be interacted with. They can, but if you work them by hand you’ll be killed instantly. Instead, shoot them from a distance and kill the enemy that appears.
Now you’re on a higher level, go through the nearest door and you’ll find a crawl space hidden next to a clay pot. Disable the trap and turn the valve, which will open the iron gate. As the enemies approach you can backtrack a little and they will walk right into the trap if you time it correctly with a well placed shot.
Turn the valve to open the iron gate again and deal with the last enemy, then grab the final Old Lithograph and head back to the monument to save the game here and upgrade. When you’re ready, place the Lithograph on the monument and prepare to face-off with Box Head.
After the cutscene sprint past the spikes and avoid the bear traps as you go. If you get snapped by the traps you’re done for. Break Leslie out of the cell, turn the valve and get ready fight The Keeper/ Box Head.
You’ll have to kill The Keeper a few times. The best technique is to hit him with a Freeze Bolt followed by a body shot from the Sniper Rifle.
When you’ve knocked him down for the first time, go to the next room and loot the table, before heading back to the main room and using the valve again. Box Head comes back, so run into the next location where he’ll be trapped in. Don’t waste bullets on him.
When he returns again hit him with the Freeze Bolt/Sniper Rifle combo, disable the bear trap, then head through the open door. As Box Head smashes the valve head to the side room, which luckily has a spare valve on it. Freeze the Keeper and fix the valve to restore oxygen.
Now sprint past The Keeper and climb the ladder in the next area, following the on-screen prompts to shake free of an attack. After the final cutscene the fight is over.
Now go forward, turn left at the waterfall and climb down the ladder to complete the chapter.