Romero's head is an Easter Egg that looks like the severed head of John Romero impaled on a pole. It appears in Doom II's MAP30: Icon of Sin. MAP30 is Doom II's final map, and the final boss (sometimes known as the Icon of Sin due to the level it appears in) must be damaged with the rocket launcher to finish the level. Players must use the rocket launcher to shoot into a hole in the monster's head; inside of this hole, the target that has to be hit is Romero's head. 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 has less health than a revenant (250 vs. 300), but each rocket will do only partial damage, so it takes at least three rockets to kill the head. When the head dies, the sound BOSDTH 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 (absolute, not relative).
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.
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.