New to DOOM Eternal is the Fortress of DOOM, a hub in which you can access missions and practice your skills.
You’ll find that the Fortress of DOOM is actually pretty sizeable, and dotted around are sealed chambers containing collectibles and other goodies such as skins for the DOOM Slayer. Yup, DOOM Eternal has skins for you to unlock.
There are two ways to unlock skins for the DOOM Slayer and other playable demons in DOOM Eternal. The first way is to complete weekly challenges and milestones, earning experience and levelling up as you go. To unlock the Classic DOOM Marine outfit as well as the Praetor suit from DOOM 2016, however, you need Sentinel Batteries.
Sentinel Batteries can be found within levels in DOOM Eternal. You can also obtain one by completing all of a level’s challenges. Once you have just two batteries, you’re able to unlock the Classic DOOM Marine skin, or another one found in the Fortress in DOOM if you prefer them.
Simply locate the skin you want to unlock in the Fortress of DOOM, and you’ll notice there are sockets on either side of the sealed chamber. Interact with the sockets and you’ll be asked if you want to insert a Sentinel Battery. Place a Sentinel Battery in both sockets and the chamber will be opened, granting you access to what is inside. To equip the Classic DOOM Marine skin or any other you’ve unlocked, return to DOOM Eternal‘s main menu and select the customise option.