"The Icon of Sin (also called Baphomet) is the final boss of Doom 2 and both chapters of Final Doom. Its image appears in the original Doom on numerous pieces of stone tablets, although it is never an enemy in the game. The Icon of Sin takes its name from the final level of the game, in which it is fought.
The Icon of Sin itself is a large demonic goat-like head set in a wall. This demon, the largest in the Doom series, continuously projects skull-faced cubes from its exposed brain. Upon landing, these cubes spawn monsters to attack the player. It cannot summon Cyberdemons, Spider Masterminds, or zombies. If a cube lands on the player, he or she will be immediately telefragged, even with an invulnerability powerup or the God Mode cheat. In normal play, the player can only defeat the Icon by shooting rockets into its exposed brain. Its size is further hinted at in part of the Doom II epilogue: "The monster shrivels up and dies, its thrashing limbs devastating untold miles of Hell's surface."
The Icon itself is not a true monster, but merely a static wall texture. Using the noclip cheat code, one can enter The Icon of Sin and see the "brain". Inside, there is a sprite of lead game designer John Romero's head impaled on a spike. It is within the splash damage radius of the rocket launcher, and damaging the head is what kills The Icon of Sin. It is not the head however that spawns the monster cubes. These are created by an invisible monster placed right behind the Icon's brain. It sends the cubes to one of about 10 spawn spots placed on the map. If no spawn spots are placed on a custom map, the Icon of Sin cannot spawn monsters. The arcane chant the Icon of Sin speaks is actually John Romero speaking "To win the game, you must kill me, John Romero!", distorted and reversed.
In Doom 2, The Icon of Sin may have been the mastermind behind Hell's invasion of Earth (in a way similar to the Spider Mastermind's involvement in the first game), and it certainly was the Icon of Sin that was bringing forth all the monsters. In the Plutonia chapter of Final Doom it serves as Hell's gatekeeper. In the Evilution chapter of Final Doom, it is simply referred to as the "demon-spitter". Whether or not this means that this demon is the devil is never explained or hinted at. Its Baphomet moniker is derived from a remark made in the Final Doom manual - that "the Spider Mastermind and Baphomet no longer seemed to threaten". Although not explicity stated, this reference to 'Baphomet' is generally accepted as being Doom 2's Icon of Sin; certainly, the monster's head resembles that of the demon Baphomet." That is the description from Wikipedia, and pretty much tells it all. It is the leader of Hell. And trust me, it is a pain to beat.