Further info on the Boss monster in Doom II:

When you first attract the attention of the Boss, the game
will play a long sound bite.  It is reversed speech, which
if played forwards would say "To win the game, you must
kill me, John Romero."

Behind the hole in the head of the "wall-demon" is a narrow
chamber.  Near the back wall of the chamber is spike rising
out of the floor, and mounted on the spike is the head of
John Romero, co-creator/programmer of the Doom series.
This chamber is normally inaccessible to the player.  You
can gain access to it through the use of the No-Clipping or
"walk-through-walls" cheat code (see DoomCode for details).

If you're facing John's head at close range, you can kill
him (ending the game) with any weapon, including your fist.