Romero's head is a thing that looks like the severed head of John Romero impaled on a pole. It appears in Doom II MAP30: Icon of Sin, where it must be damaged with the rocket launcher to finish the level. It is located in a room that cannot be reached or looked into from the outside; the only way to see it is to use the idclip cheat code and go behind the face of the final boss.
The head is deliberately placed in a small pit to prevent it from being attacked with means other than the splash damage of the rocket launcher. However, unintended exploits have been discovered that allow the player to hit it with any weapon from the lift.
The head has 250 hit points meaning it is usually killable in two to three rockets. Though in source ports that remove/fix the infinite Z height quirk from a rocket's splash damage in Vanilla Doom, it may take considerably more.
The ZDoom and Doomsday ports remove the infinite Z height from rocket splash damage but include a special exemption, via mobj flags, for Romero's head mobjs (rocket splash damage still hurts Romero's head mobj as if it had an infinite Z height).
The head alone is harmless, but usually with these heads come spawn shooters. The head roars in pain when damaged, but it is the spawn shooter, not the head, that initially speaks backwards in Romero's voice.
When the head dies, the sound DSBOSDTH is played, explosions are spawned and the level ends. The explosions are created 320 units south from the Romero's head at heights between 128 and 640 units.
Romero's head has a sprite prefix of BBRN, and is also known as the Boss Brain, or Big Brain (an interpretation by the DeHackEd author before the Doom source code was released). The former name can be taken as part of an insider joke, as the "boss brain" of the level would be the head of one of the bosses at id Software.