If you were trying to 100% the game and this final secret achievement was the only one missing, worry no more.
Black Market Farming Guide
The Green Demon Face
This “green demon face” is the entrance to the black market, it can spawn in floor 1 through 5, Oubliette and Abbey of the True Gun with an abysmal spawn rate (i’ve seen 2 in 100h -_-),once you enter the face you get the secret achievement.
First things first you’ll need to do is mod the game, called “mod the gungeon”, this is necessary for we need to use the console, the mod allows for that, link bellow with instruction on installing:
Once the mod is indeed installed, when you enter the game, instead of the name “enter the gungeon”, it’ll be “mod the gungeon”, first step done.
Second thing is having the shortcuts unlocked.
Although is possible to find the entrance in several floors, most images i saw of other people finding it were on floor 3, so i used that shortcut.
The Farm Itself
Now that you’ve gottan all the pieces of the puzzle together let’s get that achievement.
Once you spawn in the floor of your choosing open console ( with the key ~ or ) and use the command “give gungeon_blueprint” , this item will allow you to see the whole floor layout alongside secret rooms ( i don’t think you need to have this item already unlocked for this to work).
This will take a while, if you don’t find the demon head on the center of a room, restart and repeat giving yourself the blueprint item.
Until you see a floor like this
Once you do, repeat twice the command “give 50_casing” and enter the face to be free of this frustating achievement.