The Icon of Sin can be seen in The Necropolis, and is mentioned briefly in data logs. The Icon of Sin is the massive skull of a goat, embedded into a rock face, with a large bondy body leading out of the bottom of the rock face, going for countless miles into the hellscape. The bloodied visage is bound by thick rusted iron chains, and it acts almost like a overseer to demons guarding the crucible, as it is above the demonic passageway leading to the teleporter that gives access to the arena where the player faces off the Hell Guards. The skull itself is made of primarily bone, with a bloodied gaping hole in the center of what is presumed to be its forehead, another homage to the classic doom in which the center of the depression houses a large rune. Large misshapen teeth are the bottom of the Icon's upper and lower jaw encrusted with a brownish substance, and the upper teeth show signs of decay. Chains hang from the gaping hole in the center of the skull, and its eyes are milky white, almost cataract in appearance surrounded by dark red fleshy folds. The interior of the skull is filled with flesh and blood ,and some parts of it can be seen. The nostrils are wide and tall, bigger than the player model and are embellished with bone spikes on one end. The skull also has mandibles on either side, but are not attached to the primary skull. The space behind the Icon can be glimpsed partially as the player passes although it is empty and a blank space behind the skull.
A tablet retrieved from Argent D'Nur suggests that the Icon of Sin is the reborn and demonized son (or was at least demonized by the Icon of Sin) of the Night Sentinel that betrayed his brethren in return for the return of his son that was unleashed onto his father's realm during the battle that assured hell's victory of Argent D'Nur. As of now, the demon is currently dormant, according to a UAC data log that makes reference to it:
- "Imagine yourself worshiping before the Icon of Sin, in awe of its splendor even as it sleeps till the Call of Ages comes."
The Icon of Sin is confirmed to be one of the bosses of the game.