Answer: Doom II
On the surface, the final boss of the iconic video game Doom II appears to be a giant demonic head attached to the wall of the final chamber you enter. The demonic head, however, is essentially an illusion. You shoot at it and it seems to take damage, but in reality, your bullets and rockets pass right through the opening in its forehead and into a bizarre Easter egg chamber behind the head.
If you pull down the game's command console and turn on no-clipping (with the command idclip ) so that you can pass through the demonic head, you end up in a chamber where the real end boss (and the creature takes all the damage from your rockets) is located.
The creature is none other than the game's lead developer John Romero (or, at least, his severed head on a stick). To add to the strangeness of the encounter, the entire time the creature is roaring and unintelligible sentence which, when played backwards is actually Romero, says "To win the game, you must kill me, John Romero".