The Door Among Us

A mysterious door that appears wherever crowds of people gather. The door doesn't stand out to those within a certain range of it, it simply fades into the background in their perception. On occasion, the door opens for people but there are no records of where they go, but all reports of those "missing" people discover them safely at home 3 days later, with 3 new skills such as speaking a new language, learning to draw or paint, learning carpentry, cooking, sewing, and many other possibilities, though one of the skills is always combat related. After this event the walkers, people that the door has opened for before, actively notice when the door appears again. Ever since the first walkers entered the door, vague reports of people with large backpacks and suitcases disappearing into a strange door all around the world at exactly the same time.