Fortnite's Fortnitemares unleashes spooky skins, exclusives in time for Halloween
https://www.cnet.com/news/fortnites-fortnitemares-unleashes-spooky-skins-exclusives-in-time-for-halloween/
This skin changes throughout the game.
There are other in-game exclusives for the event like special weapons (Six Shooter and Fiend Hunter Crossbow) and limited interactive skins that change as you deal more damage to enemies.