DOOM: Eternal has a variety of different skins that your Doom Guy can wear during its solo adventure. You have a handle that you can get through the doom fortress, and some of them can be won by playing the game during the life of the series by reaching the level 10. There are several skins similar to Zombies for the Doom Guy and several demons, you can play online.
To get these skins, you must join the Bethesda Killer Club online. You can reach the Club on Bethesda website and you can start upgrading your account for free. All you have to do is play doom: Eternal when you are connected to the account.
Several awards are available. The first reward you receive at one is the Zombie Doom Slayer skin for your Guy doom. You can play there in the custom screen customization menu. Later, you can possibly win zombie skins for the Manububus and a skin of Imperial Marauder, and the final level 15 reward is the appearance of the 25th anniversary killer.
Make sure to connect to the Bethesda website to start earning experience points. There are 15 levels in total to win.