|Icon of Sin|
|Type||Giant,Teleporting citadel boss|
|Signs of nearing||Screaming|
|Theme song||Final boss music|
In Slender Fortress Edit
The Icon of Sin is the Last boss encountered by picking up the last soul key. It spawns a potentially endless series of monsters of demonic ilk (minus cyberdemons, spiderdemons, and zombies), all of which count towards the player's end-of-level kill percentage; as a consequence of this, the player can finish the level with a greater than 100% kill percentage.