Answer: Doom II
On the surface, the last boss of the iconic video game seems to be Doom II huge demonic head attached to the wall of the last chamber. The demonic head, however, is essentially an illusion. They shoot at it and it seems to take some damage, but in reality your bullets and missiles pass right through the opening in the forehead and into a bizarre easter egg chamber behind the head.
If you pull down the command console of the game and turn on no-clipping (with the command idclip ) so you can go through the demonic head. You end up in a chamber where the real boss (and the creature that takes all the damage off your missiles).
The creature is none other than the main developer of the game, John Romero (or at least his severed head on a stick). Adding to the strangeness of the encounter is that the creature roars an unintelligible sentence all the while, which, when played backwards, actually tells Romero, "To win the game, you have to kill me, John Romero."