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 story as Baphomet. 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.
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.
- Every monster spawned by the final boss counts towards the player's end-of-level kill percentage. However, they are not counted towards the total amount of monsters in the level, relatively to which the said percentage is calculated; this is also the case with arch-vile resurrections. This can result in a kill count going over 100%, although in the presence of the final boss the result can be far more severe. For example, if the level contains only one monster or none at all (e.g. MAP30 of Doom II), every monster spawned by the final boss adds whole 100% to the kill percentage. To remedy this, certain source ports employ "smart" kill counts, which account for boss spawns and enemy resurrections.
- 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 (explosions just before level ends)
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?"
- In Doom II RPG, a similar entity also has its upper body visible, revealing a biomechanical bulky build attached to a wall. A virtual version of it can also be found (known as VIOS) which has arms and goat-like legs.
- Reversed, the DSBOSSIT 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 (DSBOSPIT) 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 MARBFACE 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 Baphomet, an archetype of the pagan Horned God.
 See also
- (0,0) respawning bug
- Spawn cube targets not preserved in saved games
- Spawn cubes miss east & west targets
- Lopsided final boss explosions