The first of the game's Cheat Codes can be found near the end of the level. Walk through the corridor in the image below, and you'll see a yellow question mark hidden behind a gate above you. Continue ...

As soon as you start Doom Eternal you will be immediately dropped on to earth, ravaged by a demon invasion by 3 hell priests. Luckily you are already on the tail of one, you are the doom slayer after ...