Where can I find Deadpool's letter?
When you are in the Battle Pass menu, you have to look carefully on the right of your screen and locate the air vent. Then you have to click on it. You have arrived in the landmark of Dead Pool ! Then, to validate the challenge, you just have to look for the letter, which is, fortunately, not very complicated to find.
As you can see in the image below, the letter is on the ground, at the bottom left of your screen. On it we can read that Dead Pool asks Epic Games to change the design of his bus, in which case he will cut his arm.
Once the letter has been retrieved, your challenge is validated. You can go back to the previous screen, to complete the next challenge.
You can find all the Deadpool challenges by going to this address.
