In a building near Colt's Updaam Flat in Deathloop you will find a smoking device. You'll learn that three Eternalists have been having a competition, with the last one to pass out from smoking getting access to what's in the back room. There are three codes to find, and you'll need to do some light sabotage to get them. Here's how to complete the Pact of Smoke side mission, including details on finding the three key codes, and what's in the backroom.
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Deathloop Pact of Smoke Walkthrough
There's a building joined by the courtyard outside of Colt's Updaam flat in Deathloop. You can go inside and find a strange smoking device. Look for a note next to it which will give you a code from someone called Cass. It seems you will need three codes to unlock the backroom. There's a note on the other side of the room, next to the console shown in the image above. It reveals that the game is set up so that Cass wins every time, the green light corresponding to Cass' safe yellow tube currently. Press the button here to switch the green light to the top row, the red pipe. You've now set it so that instead of Cass surviving the gas, another of the remaining two Eternalists will. Make sure you are doing this in the morning, ensuring that the red tube is safe before you leave. You will then visit Karl's Bay in the Afternoon to find an Eternalist called Vanya.
Note that you will need to complete this loop twice in order to get all of the codes. Switching to the red pipe will allow you to go after Vanya. You will then need to revisit the Pact of Smoke in the morning and switch to the blue pipe, before going after Anatoly. Let's start with Vanya.
Vanya Location and Code
Head to Karl's Bay in the afternoon. You'll start in the tunnels, take the exit on your left. You should see the 'Gardens of Perception' archway in the distance, this is to the from the door that you exit the tunnels out of. Go through the archway.
To your right, you will see a set of stairs leading down to a couple of enemies. Kill them and look for a nearby building. If you set the red pipe to safe that morning, the door will be open. This is Vanya's flat.
Vanya will be inside. Kill her and then look for a note. This will reveal her code. You now have two codes for the Pact of Smoke. All that's left is to go after Anatoly's code. You will need to start a new loop and head to Updaam in the morning. Switch the button in the Pact of Smoke room so that the blue tube is the safe one.
Anatoly Location and Code
Anatoly's Flat can be found in Karl's Bay in the Afternoon. Look in the area around the back of the two hangars. You'll find a sign that reads 'Treasure of the Ice'. Anatoly's flat is to the left of the entrance, decorated with green lights. If you set his smoking tube to safe that morning, you will be able to get inside his flat. Kill him and then look at his note to find a code. You now have all three.
NOTE: There's a reason that we aren't listing the codes in this guide. They are different for every player, so we are unable to simply list the codes, you will need to find them yourself.
Pact of Smoke Backroom Rewards
Using all three codes to open the Pact of Smoke backroom will get you a glimpse into some of the seedier aspects of Blackreef. There's an Evangelist locked inside what looks to be some sort of dungeon. I'll leave that up to your imagination. There's also an audio log featuring the second part of Harriet's interview with Julianna.
That's everything you need to know about the Pact of Smoke in Deathloop. Head here for our guide on opening the hidden glyph safe in Updaam.