Quick Answer
Reach Deimpo Mausoleum through the face barrier near Boudin; your goal is six hidden symbols, then the boss near Chef. Burn the book first to obtain the acid vial for your basic throwing daggers. Inside, you need a special mask and its lens to interact with the symbols in the dark tunnels.
How do you reach Deimpo Mausoleum?
Use acid-coated basic throwing daggers to clear the face barrier near Boudin in the Tower of Knowledge.
Burn the book for the acid vial
Burning the book gives you the acid vial, which coats your basic throwing daggers with acid. You can then remove the face barriers throughout the tower. If you are still working through that area, use the Tower of Knowledge walkthrough.
Find the barrier near Boudin
Return to Boudin, who is dead by this stage, and clear the nearby face barrier. The opening leads toward the mausoleum. For more on the weapon type used here, see the throwing weapons guide.
Open the early campfire shortcut
This route also lets you open the door near the campfire characters you met at the beginning. Those characters are also dead by this point.
How do you open the mausoleum and reach its boss?
Use the switch beneath the big stairs, then interact with six hidden symbols to access the boss near Chef.
Look under the big stairs
Find the switch beneath the large staircase, near two skeletons. Use it to open the mausoleum.
Prepare the mask and lens
You need a special mask you can equip and a lens that goes with it. Their exact item names and pickup locations are still being added here.
Search the dark tunnels for six symbols
Equip the mask with its lens and interact with all six hidden symbols throughout the tunnels. All six are in the dark area, where the magical residue is easier to see. Individual symbol positions and the complete junction-by-junction route are still being added here.
Head to the boss near Chef
After interacting with all six symbols, you can fight the boss near Chef.
The opening switch and the six-symbol requirement are separate parts of this route. If you have opened the mausoleum but cannot fight the boss, check your symbol interactions.
What can you pick up along this route?
You get the acid vial before entering; the mausoleum's exact reward list is still being added here.
The mask and lens are required for the tunnel symbols, but their pickup locations are still being added. You also gain access to the door shortcut near the early campfire.
Acid lets you clear other face barriers in the Tower of Knowledge, revealing secrets and weapons. For other concealed routes, see the hidden passages guide.
Where do you go after Deimpo Mausoleum?
Before the boss, your next destination is Chef's nearby boss encounter after you interact with all six symbols.
The destination after that boss is still being added here. Use the Queen's Domain walkthrough to return to the wider progression route.
FAQ
Burn the book to obtain the acid vial, then clear the face barrier near Boudin with basic throwing daggers. That opening leads toward the mausoleum.
You can find it beneath the big stairs, near two skeletons.
You need to interact with six hidden symbols in the dark tunnels using the special mask and lens. Look for their magical residue in the dark area.
Interacting with all six lets you fight the boss near Chef.