Final Boss

''This article is about the original Doom Games boss. For the Easter egg in Doom 4, see Icon of Sin (Doom 2016).''

The Icon of Sin is the final boss encountered in Doom II. See this walkthrough for a gameplay description of the boss in its original level.

The Icon of Sin appears as a massive, goat-like biomechanical head on a wall, with a surgically exposed brain that allows it to spawn endless scores of demons. Although only its head is visible, the endgame text indicates that the entity has a gigantic body as well.

In Doom II, the boss is not referred to by any specific name. The Doom II Official Guide refers to it as the "Dark One". It is known in the Final Doom manual introductory stories as "Baphomet" and "Gatekeeper". In the text screens of TNT: Evilution and The Plutonia Experiment it is called demon-spitter and Gatekeeper respectively. The graphic and sound files refer to it by a series of different names: RWDMON ("wall demon"), ZZZFACE ("face" with a prefix for sorting purposes), BBRN ("Boss Brain"), DSBOS (" [Doom sound] Boss"). In 2016, John Romero revealed to a Twitter user that Icon of Sin was the 'official' Doom II boss name since 1994.

The final boss is not an actual monster in the technical sense, since it does not count towards the monster kills percentage at the end of a level, and is not affected by the command line parameters that affect standard monsters. Additionally, monsters are a single Doom thing, but the final boss is made up of a number of things:
 * Romerohead sprite.gifo's head is the thing that must be destroyed. It starts with 250 hit points, but it is hard to hit due to its location deep in a hole in the boss's head, therefore it can only be damaged and killed by the blast radius of a rocket when playing levels that put the monster in the original location behind the wall. However, modern source ports allow for aiming freely and so can allow direct hits with any weapon;
 * A monster spawner in front of the head launches spawn cubes;
 * Several spawn spots are located around the map. These are where the skull adorned cubes land and spawn a monster;
 * Wall textures ZZZFACE1 through ZZZFACE9 are arranged to construct a demon face with a hole into its brain. This is what the player sees as the boss, but it has little to do with its functioning.

Details
It spawns a potentially endless series of monsters of demonic ilk (minus Cyberdemons, Spiderdemons, Nazi soldiers and zombies), all of which count towards the player's end-of-level kill percentage; as a consequence of this, the player can finish the level with a greater than 100% kill percentage.

The sound effects associated with the Icon are:
 * DSBOSSIT (wake-up noise)
 * DSBOSPIT (cube is fired)
 * DSBOSCUB (cube in flight)
 * DSTELEPT (cube turning into monster)
 * DSBOSPN (Romero's head takes damage)
 * DSBOSDTH (Romero's head dies)
 * DSBAREXP</tt> (explosions just before level ends)

Background
A quote from Robert Prince:

"It was a late night and the walls were shaking at id Software. Why? There could be only one reason -- Romero is in the building! Otherwise, it was a quiet, unassuming office -- better yet, a library. Then things quietened down, and I supposed that Romero had left. In fact, everyone but Romero had left, as I discovered when he came into the room I was using for "sound development." He sat down next to me and said that we needed a sound for the final boss to make when a player enters that level. I said that I had some possibilities roughed out and since he was there we could plug them into the code to see how they'd work. We went into John's office to look at the level (he had the only 21" screen). While he was whizzing around the level, all of a sudden he said, "Wait, what's that?" He had clipping off, which means that he could walk through otherwise "solid" objects. He had walked into the wall where the final boss head was attached. Lo and behold, there inside the brain of the boss was Romero's head on a stick! We both laughed a while and Romero decided that the artists (Adrian Carmack and Kevin Cloud) had put it there as a joke. As it turned out, John Carmack had programmed the code so that Romero's head was the object that a player had to hit in order to kill the boss. And this head was down a shaft inside of the wall so it was normally out of sight. It was at that point that Romero and I decided to record his voice and use it as the final boss sound. We went back into the sound room and John started saying different things in a very pumped up voice. He finally said, "To win the game, you must kill me, John Romero." I took that phrase and put some phasing on it and then reversed it. Shades of the rumors of "Satan" on different pop recordings! We decided not to tell anyone else what it said. We had the fun of seeing the artists' expressions when they first entered the level with this sound going. We made them sweat a long time before we played the phrase in its original form. Can you tell that we always had a great time doing this stuff?"

Trivia

 * It was designed in the general image of the pagan devil lord Baphomet, a goat-headed demon.
 * The fact that the monster-generating cubes spawn directly from the Icon's forehead opening is reminiscent of how Satan gives birth to his daughter Sin in John Milton's Paradise Lost in that she is born out of his head (a parthenogenesis originally based on Zeus and his own daughter, Athena).
 * The reversed recording of Romero's voice is deliberately reminiscent of how rock and heavy metal bands such as Led Zeppelin and The Eagles have been accused by religious fundamentalists of using a process known as backmasking to include hidden, reversed messages praising Satan in their music.
 * An image of the final boss is also displayed in the earlier MARBFACE</tt> texture.
 * The launch sound of the spawn cube (DSBOSPIT</tt>) comes from Sound Ideas' General series sound library: Disc 6015, track 28-1: "Fire,Ball - Impact and large fire burst, rumble." It is a very widely used sound effect that has also been used in several movies (such as Dragonheart) television series (for example, ReBoot and Xena: Warrior Princess) and documentaries (like The Hawking Paradox from BBC Horizon).
 * On Doom ports that allow for mouselook, the end boss becomes almost trivial in difficulty. The player is able to stand at the teleport landing for the main room and simply fire rockets up into the boss's exposed brain.
 * An homage to the Final Boss' greeting message appears in the source files for Half-Life: Opposing Force. The file is dsbossit.wav, the same name of the sound file used by the Icon of Sin. In its original pattern, it's gibberish, but when reversed, it states: "To win the game, you must kill me, Randall Pitchford". Randall "Randy" Pitchford is the CEO of Gearbox, the developers of the Opposing Force expansion to Half-Life. It is unclear whether this sound file actually plays in the actual game.
 * In Doom II RPG the main antagonist is VIOS (Virtual Icon Of Sin).
 * The Icon of Sin makes an appearance in the 2016 reboot, in the level The Necropolis. Shooting the exposed forehead with a Rocket Launcher will cause a cube to spawn, and a collectible will be revealed where it lands.