Dwarf wears what dwarves hidden challenge
Like many hidden challenges, the latter is not very difficult to achieve. To validate it, you will have to go to the south of the map (see below for the exact location) in order to reach dwarves in full event. The latter actually seem grouped in bleachers, and are waiting for something important.
No particular action is required on your part, except to go there. After a few seconds, the challenge will be validated, allowing you to unlock 25 XP. A significant sum to unlock all the content of the Battle Pass. All the challenges of the latter are also to be found at this address.