Bring down the console with tilde [ ~ ]. Then type these:
save: Make a save game.
load: Load a saved game.
kick [nick]: Banish a user from the current network game.
pause: Pauses the game.
quit: Quits the game.
say: Communication with your teammates.
soundinfo: Information about your Sound System.
status: Information about current game.
version: Shows your game's version.
Look closely at the nailgun ammunition boxes to see the "NIN" logo for Nine Inch Nails, the band that did the music for the game.
In Episode 2 Map 7 (secret level), shoot all of the gargoyle faces once. They will bleed. Then, go to the long, flashing corridor where you encounter two Hell Knights in alcoves. Shoot the wall of the second alcove to reveal a hidden room containing the initials of Tim Willits and Teresa Chasar, along with a raven symbol for the raven levels that they made for Doom 2.
This is a secret extra for the Crypt Of Decay (Episode 2 Map 3). After the spiked hall, go forward, turn backwards, then look to your right. There is a room there where you must take a leap of faith to cross. Get the quad and keep going towards the wall until you reach a hall. Enter it and go down and swim. The Dope Fish is at this location. Go to the left of the fish and shoot to find the exit to the secret level.
When on the House of Chthon level (e1m7) type in the following cheats: God, Noclip and impulse 9. Now move forward, get the rune and go through Chthon. Keep going until your out of the level but you see a big black box. Go into the box and 12 Shamblers and 8 zombies will attack you. Kill them all, then kill Chthon and complete the level. When you do you will notice that their will be no explosions of blood, that is because you have killed all of the monsters!
Press the tilde [ ~ ] weapon to bring down the console. Then type these codes:
god: God mode.
noclip: No clip mode.
impulse 255: Quad damage!
impulse 1: all weapons
kill: suicide [only for masochists]
fly: fly mode on
notarget: only if you shoot enemies they will shoot you.