Difficulty: Legendary
Level: Sacred Icon
Effect: Unknown

After coming down from a piston, about halfway through the level, you’ll come to a room completely filled with Flood fog. Go into the room and you’ll see a stack of crates. Jump on top (while cloaked would probably work better, so the Flood don’t notice you), and across to the next highest level. If you look in the last hole in the wall, you’ll see the skull in with a dead Flood.