There is a switch that causes an elevator tower to rise in front of the demon face that the player can stand on to fire the rockets into the hole. The only way to damage it is to fire a carefully-aimed rocket into the hole in its forehead. The Icon of Sin is impervious to all the weapons in Doom, including the BFG. There is therefore a time challenge aspect to defeating it: as the player attempts to battle the Icon of Sin, the arena becomes overrun with hellspawn until there are too many to defend against. When it arrives at its destination, the cube transforms into one of the many monsters encountered on previous levels. This hole is the source of the challenge: every few seconds a spinning cube emerges from the hole and flies toward one of several locations within the arena. In the middle of its forehead is a gaping hole, the flesh apparently pinned open. It consists of a giant wall with an image of a goat-like horned demon on it. A teleporter then takes you to a large empty arena with slime pouring down the walls and the monster facing you. The level starts with a small room filled with weapons and powerups (which is always a worrying sign).

The Icon of Sin is the final boss of Doom II: Hell on Earth, encountered on the last level of Doom II (MAP30).

Icon of sin doom 4