One of them will channel your inner Yugi Muto.
Here's how to become a Shadow.
How To Become A Shadow In Fortnite
This challenge is extremely straight forward and requires no effort on your part.
First, you'll need to load into a regular BR game (solos, duos or squads).
Of course, you'll need to go through the formalities of getting onto the map itself. So drop down as fast as you can in a populated area.
The Authority, now known as 'The Ruins' seems to be where everyone congregates.
All you need to do is find an enemy and let them kill you.
You'll respawn as a shadow and should see progress for this challenge start to rack up.
Once done, you can feel free to leave the game. Do this three times and you'll bag 40,000 XP for your troubles.
While this should take three games to complete, I did encounter a bug where it didn't register in one game.
However, due to the extremely easy nature of this challenge it shouldn't take long to do even if it does have a hiccup or two.