The Icon of Sin is the final obstacle in the Space Marine's way, the final boss of Doom II: Hell on Earth. This monster is the vile culprit behind the invasion of Earth and must be killed to stop the invasion. To defeat The Icon you have to shoot the demon in the forehead to deal damage. However, this is a very hard target to hit so the best weapon to use is the rocket launcher. To fight back the Icon can spawn monsters all over the map by shooting skulls at the spawn points from his forehead. However, this boss contains a dark secret.
Big Boss Head
The Icon of Sin itself is actually just a very large wall texture; rockets fired into the exposed brain go into an invisible gap and strike the actual character sprite for the boss, and because there's no way to actually see that sprite without cheating, as a joke, the developers made it John Romero's severed head on a stick. In fact, the ominous gibberish the Icon chants as you first enter his chamber is not some ancient demonic language, it's John Romero saying "To win the game, you must kill me, John Romero!", with the audio given alterations to sound demonic and then reversed.
The Icon can spawn infinite amounts of enemies. So, the player can have over a 100% enemies killed statistic. He can also spawn any enemy in the game minus Cyberdemons, Spider Masterminds, and all varieties of Former Humans (the former two because it would be unfair, and the latter because it cannot create them, merely possess them).
Here are the sound effects associated with The Icon of Sin:
(cube is fired)
(cube in flight)
(cube turning into monster)
(Romero's head takes damage)
(Romero's head dies)
(explosions just before level ends)
Inspirations for the Icon
The icon has a couple inspirations for it. First, the Pentagram which is usually associated with Satan/the Devil and looks like a goat's head which resembles the Icon's look. Also, Satan inspires the idea of monster's being spawned out his head and the idea that he is the mastermind from hell.