A secret item you can find throughout Doom: Eternal are cheat codes. You can use these cheat codes when repeating missions in the game to alter how you play them, such as having infinite bullets or having access to a certain power-up at all times. All of them are hidden, though, and some are difficult to locate.
You will have access to this cheat code’s location when you enter the sewer location, immediately after jumping a ravine to access a subway car. Here, you will find tentacles for the first time and have to deal with them in this area. Follow the game’s pathing until you have to go left, and there’s a block on your right with nowhere else to go.
You need to jump onto the block to gain access to a ventilation shaft. The shaft is blocked by a destroyable vent, which you can quickly melee. On the other side is the first cheat code. Grab it and the path previously blocking this location will open up for you. You will land close to where you started in the sewers and can turn to make your way back through your path.
There are 14 cheat codes you can find in total. Keep your eyes out for them on your map, and remain vigilant on their location.