To reach Duat, you will need to visit the Black Market and the City of Gold, two additional hidden areas that are difficult to access. You will need a series of artifacts; the Eye of Udjat, the Hedjet, the Ankh and the Scepter - simply to reach the City of Gold and sacrifice yourself. Reaching the Cité d'Or is the real challenge here. Well, finishing Duat is the real challenge, but let's not get too far ahead of ourselves.
Here is the ridiculously complicated process you have to go through to reach Duat. To beat him? You are alone there, bub.
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How to reach Duat | Secret location guide
Duat's secret area is deeply important - and ridiculously difficult to reach. You'll need four Artifacts to get there, and you'll need to start your run from the Residence each time.
Duat is a strange world between life and death, where you will encounter two optional bosses. Instead of starting at the top and working your way down, you'll start at the bottom and work your way up. The ropes don't work here - and you'll have to drop any held items to gain access to the location. Only upgrade items, bombs, and ropes follow you into the new area.
The good news is that the ropes can be collected after being thrown, so you can use them as weapons and never run out.
Step # 1: Get the Udjat Eye & Hedjet
Eye of Udjat is a special item acquired over 1-2 or 1-3. Look for a door leading to a locked chest. The key is somewhere on the map. Usually, it is quite easy to acquire.
Then use the Udjat Eye to find the secret location of the Black Market. The Black Market Gate spawns in the Jungle - 2-2, 2-3, or 2-4. When you are near the door, the Udjat eye glows and blinks. Bomb there to reveal the door. You will find the Hedjet article on the black market.
Step # 2: Obtain the Ankh and the Scepter
Then you will have to progress deep into the dungeon. Reach Olmec (complete 2-4) and go down to the back of his room. In the lava, there is an optional door leading to the caves. Follow the path to retrieve the Ankh. You will need it - the Ankh gives you a second chance.
Go even further to 4-1. Here you will find Anubis at the temple gate, a special boss type enemy. Defeat him to acquire the scepter. You now have everything you need to reach the secret area of the City of Gold.
Step 3: Reach the City of Gold and sacrifice yourself
The gate to the City of Gold will appear in 4-2. Use the Scepter and the Hedjet to access the City of Gold - this golden city is full of silver, but we are looking for the sacrificial altar in the center. There is a large altar which is quite suspicious.
Descend from above to make sure you die on the altar. Dying on the altar sacrifices you - and sends you into the Duat.
Sacrificing yourself turns you into a ghost version, removing your owned items but retaining any upgrades, bombs, or ropes. In this area, you will meet another boss Anubis and Osiris. Defeating Osiris will reward you with a mysterious stone tablet and teleport you to the Ice caves.
What is the stone tablet for? We don't really know yet, but we'll update this guide with more information as it's discovered!