The Icon of Sin is an easter egg in the PC version of Daikatana.
It is a reference to John Romero's head being the final boss of Doom 2 - behind the Icon of Sin, is John Romero's head on a stick, which you must kill to win the game. It uses the reversed sounds of John Romero saying "To win the game, you must kill me, John Romero!" Upon entering the easter egg, a reversed voice will play, which when played forward will say "Bow before the Lord of Darkness and drink from the well of Daddy's milk." The Icon of Sin easter egg does not count as a secret. However, there is a secret nearby the hidden entrance with the Ring of Fire Protection.
The easter egg is in the map E3M3A, the first level of Dungeon. To get to the easter egg, proceed through the level past the destroyable wall, obtain the Ballista and Ballista-Jump across the lava and towards the area where the Dwarf is. On the wall should be a small triangular indent, pointing near a wall. Attack the wall around it with the Daikatana. You will know which wall is correct because it will not leave the bright blue markings when you attack it. After a couple attacks, the wall will crumble and it will lead way into a hidden area.
The easter egg consists of a stylized face of John Romero with surrounding skulls. There is a bowl of milk right in front of it - and health flasks inside of it. The milk has the effect of freezing water and will inflict freezing if you stand in it too long. There are two pillars on the left and right, and cracks in the floor revealing holes, below which is lava.