The Icon of Sin is the final boss of Doom II: Hell on Earth, encountered on the last level of Doom II (MAP30). The level starts with a small room filled with weapons and powerups (which is always a worrying sign). A teleporter then takes you to a large empty arena with slime pouring down the walls and the monster facing you. It consists of a giant wall with an image of a goat-like horned demon on it.
In the middle of its forehead is a gaping hole, the flesh apparently pinned open. This hole is the source of the challenge: every few seconds a spinning cube emerges from the hole and flies toward one of several locations within the arena. When it arrives at its destination, the cube transforms into one of the many monsters encountered on previous levels. There is therefore a time challenge aspect to defeating it: as the player attempts to battle the Icon of Sin, the arena becomes overrun with hellspawn until there are too many to defend against.
The Icon of Sin is impervious to all the weapons in Doom, including the BFG. The only way to damage it is to fire a carefully-aimed rocket into the hole in its forehead. There is a switch that causes an elevator tower to rise in front of the demon face that the player can stand on to fire the rockets into the hole. However, there's an additional challenge due to the fact that the elevator is slightly too tall and it is not possible to aim downwards in Doom. The rockets must therefore be fired just before the elevator reaches the top, or just after leaping from the top of the tower. Three rockets are enough to kill the monster.
The Icon of Sin is a great example of Adrian Carmack's artwork. The demon head is integrated into the wall by a system of pipes and machinery, making it appear as though it is part of some kind of gigantic machine. It forms the largest single piece of artwork in the Doom games: it is so large that due to the limits of the game engine, it had to be split into nine separate pieces which are assembled together in a 3x3 grid. Each piece is a 256x128 texture, which in the context of the game is around the same size as a London double decker bus.
Images of the Icon of Sin are seen throughout the game - not just in Doom II, but as early as the first episode of the first game (E1M8 features a teleport gate with the image of the Icon of Sin on it). The fact that the image of the monster is woven throughout the game helps to emphasize its importance and power.
Within the game itself, the behavior of the Icon of Sin is implemented from several separate components. The first is a "boss brain" object that is hidden inside the demon head, and exists purely to take damage. When a rocket is fired into the hole, it hits a wall, and the splash damage from the explosion damages the boss brain. Once it takes sufficient damage, the death sequence is triggered, where a large number of explosions occur all over the demon face.
A separate "boss spitter" object is used as the source of the spawn cube projectiles that are launched out of the hole. This is also used to "see" when the player enters the arena and trigger the monster into waking up. The boss spitter launches the spawn cubes toward spawn spots that are located around the level.
The boss brain is the source of Doom's most infamous easter egg. Members of the Doom development team played a prank on John Romero by inserting his head on a stick into the game as the sprite for the boss brain. By using the IDCLIP cheat to go through the wall it is possible to see the sprite. Romero had the last laugh, however. After discovering the sprite, he inserted into the game a backmasked, distorted recording of himself saying, "To win the game you must kill me, John Romero". This is the sound that the player hears when first teleporting into the arena with the Icon of Sin.
The boss monsters in Doom 1 are challenging, memorable and satisfying to defeat. Episode 1 pits the player against two Barons of Hell (known internally as "The Bruiser Brothers"), which at that point in the game are the most powerful monsters encountered, and a significant challenge with the limited weapons available. Episode 2 consists of a 1-on-1 rocket battle against the Cyberdemon, probably the most memorable of Doom's boss battles. Episode 3 (and 4) pits the player against the Spider mastermind, a giant chaingun-wielding brain on mechanical legs.
Obviously the Id guys needed something to top this, and of course the obvious way to do that is to make a bigger monster. I can imagine the vision they might have had, the Doom guy coming up against a gigantic towering demon, the mother of all demons, endless smaller demons coming out of a hole in its forehead to attack the player. So gigantic that any conventional weapons, even the BFG, would have no effect on it. The only chance to defeat it, a carefully-aimed rocket shot into its exposed brain. An epic and legendary boss battle to conclude the previous 29 levels of carnage that the player had fought through.
Unfortunately, when it comes down to the execution, it all falls rather flat. The biggest problem is that it is very obviously just a scary looking face on a wall. Because of that it's very difficult to believe that you're really fighting against a gigantic monster. There is no sign of the rest of its body anywhere, it's a static image that never changes, it's obviously flat and attached to a wall (the normal Doom monsters are flat too, but the fact that they aren't attached to a wall means that you don't notice).
The challenge posed by the monster is also as laughable as the monster itself. Clearly the point is that the rocket shot has to be carefully aimed, otherwise you could just stand on the pillar, fire off a few rocket shots and be done with it. But fact is that if this was a real scenario, there's no reason you couldn't do exactly that. The entire challenge of the battle stems from the fact that you're unable to aim downwards. So what is intended to be an epic boss battle actually becomes a rather comical demonstration of the limitations of the Doom engine.
I don't think the idea itself is necessarily a bad one, it's just that everything about the Icon of Sin seems rushed and clumsily executed - from the apparent budget-saving trick of only showing its face and not the rest of its body, the challenge of the battle itself, even the cheesy sound effect when it launches a spawn cube. I guess the most surprising part is that Team TNT didn't make any further effort to improve on it when they made Final Doom.
There are various ways that the Icon of Sin might have been improved. The simplest would have been to make the face animate. Even a change of expression whenever it launched a spawn cube or was hurt by a rocket would be enough to make it clear that you're fighting against a real monster and not a wall texture. More effort to depict its body would also have helped.
A recently released megawad named Plutonia 2 gives a good example of how the Icon of Sin could have been done better. When first appearing, the demon head rises up from the ground to confront the player. Layered texturing is used so that the head does not appear completely flat, and it is shown to have a body (although it is not completely visible). The gameplay is improved as the player must maneuver into a position to attack that makes more sense.
- MAP30: Icon of Sin, Doom wiki: http://doom.wikia.com/wiki/MAP30:_Icon_of_Sin_(Doom_II)
- Little Known Fact #4 DOOM II Final Boss - Bobby Prince, http://rome.ro/lee_killough/history/doom2boss.shtml
- Plutonia 2, http://www.doomworld.com/idgames/?id=15550