Final boss

''"Icon of Sin" redirects here. For information about the Doom II map, see MAP30: Icon of Sin. For information about the entity in Doom (2016), see Icon of Sin (Doom 2016).''

The final boss (often informally called the Icon of Sin because of the Doom II level of which it is a part) is the final boss of Doom II, in MAP30: Icon of Sin.

It appears as a massive, goat-like biomechanical head on a wall, with an 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. It is referred to in the Final Doom manual's introductory stories as Baphomet and Gatekeeper. In the text screens of TNT: Evilution and The Plutonia Experiment, it is called the demon-spitter and the Gatekeeper respectively. The graphic and sound files refer to it by a series of different names such as "wall demon", "face", "boss brain" or "boss".

A corresponding enemy character in Doom II RPG is called the Icon of Sin, and has a virtual avatar known as VIOS, which has taken control of the UAC's computer systems.

Setup
The final boss is not a monster in the technical sense, since it does not count towards the 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 these:
 * Romero'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, 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. This 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).
 * 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.

Tactical analysis
It spawns a potentially endless series of monsters of demonic ilk, except for boss monsters (cyberdemons and spiderdemons), lost souls, and all three kinds of possessed humans. See this walkthrough for a gameplay description of the boss in its original level.

Background
A quote from Robert Prince:

Other games
The Doom II final boss has become a recurring character in the series, usually under the name "Icon of Sin."

Doom II RPG
In Doom II RPG, a similar entity can be found with its entire upper body visible, revealing a biomechanical bulky build attached to a wall. A virtual version of it known as VIOS, which has infected the UAC's computer systems, can also be encountered frequently within the game. It forces the player to make decisions which will affect the outcome of the game and the strength of the creature when it is finally faced one-on-one in combat. Its digital avatar has arms and goat-like legs.

Doom (2016)
In the 2016 Doom, the Icon of Sin appears as a dormant demonic entity which can be found in the Necropolis in Hell. It was previously used as an infernal weapon of mass destruction against the people of Argent D'Nur during the end stages of the demons' battle to conquer that realm. As an Easter egg, it can be provoked into firing a spawn cube just like its Doom II counterpart, which leaves behind a collectible item.

Trivia

 * Reversed, the is a distorted recording of John Romero saying "To win the game, you must kill me, John Romero!".
 * The launch sound of the spawn cube 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).
 * The final boss originates from a sketch that Adrian Carmack drew for the original Doom game. The picture made it into the texture, and was later used as the basis for the ultimate opponent in Doom II. This image of a goat circumscribed within a pentagram is a European occult tradition commonly associated with the demon or deity referred to as, an archetype of the pagan Horned God.
 * The Freedoom equivalent of the DSBOSSIT sound is a reversed and pitch-shifted recording of Freedoom contributor Fredrik Johansson saying "My name is Fredrik Johansson. I come from Sweden".