The actual creator of Doom's final boss.

In Doom II, the creator inserted himself in the game. Use the level skip cheat code, and warp to the final boss, type in the clipping code and run into the final boss, once inside you will see a guy's head on a spike. Once here, you cannot save, the window will shut down, and the file will not open at all. This is one of the original staff members. To beat this monster, shoot it with the BFG.

The 'boss' will say some kind of weird demonic voice as well, but the voice is speaking in backwards. but it says this if played forward: "To win the game, you must beat me, John Romero". John Romero is one of the creators in Doom II.