You can alter your Doom guy’s appearance in Doom: Eternal to change how he looks during your campaign and cutscenes. To access several of them, you need to unlock specific locations of your main ship, the Fortress of Doom. One of the notable armors you can unlock on this ship is the Sentinel armor appearance.
To find this armor, you need to start on the first floor of your ship, where you initially load into the game. From here, proceed to the left and go down the stairs to the second floor. Go around the corner, and continue going left through the series of two, slow opening doors. On the other end of the final door, you should see a broken pathway you can jump across and make it to the other side. The Sentinel Armor is available there.
If you want to have access to this armor, you need to use two of your sentinel batteries to acquire it. You can obtain these from any of the missions in the game, or by completing the three challenges of any mission. You can use cheat codes to make your journey easier on the mission select screen. After you have you acquire those two batteries, return to this entrance to unlock the armor. If you want to wear it, you need to go to the main menu of your game and go into the customization options to alter your Slayer’s appearance.
There are various secrets and hidden encounters scattered all over Doom: Eternal. Make sure to find them, and to also unlock the Classic Marine and Preator armor customization options also found on the Fortress of Doom.