The Siren is a boss enemy in the late game. It is fought in the area after the elevator ride with Sotiris. The player must find and kill three Talking Brains, then fend off the siren after each brain is destroyed, eventually having to kill it in the last area. This serves as a boss fight and puzzle. It is a very big enemy, often blocking the player's path. It's fast, but can be easily outrun by the player.
All of the notes near the talking brain that summon the siren are references to ways to kill it in the given parts: e.g. the first, being electrocution; the last getting crushed.
The Siren resembles a crawling humanoid with large hands. It lacks a nose, instead having an exposed nasal aperture. It also does not have eyes, possessing two horizontal, feminine-looking mouths permanently smiling. The Siren does not have legs, having to crawl to chase the player.
Area 1, The Nasty Old Man
After the player kills The Talking Brain, they see a vision of Virgil summoning The Siren from the depths. After that, it will start pursuing the player. The player must lead The Siren across the bridge towards the breaker switch, activating it as The Siren is in the water, shocking it and causing it to retreat.
Area 2, The Nice Old Dog
After killing the second talking brain, the player gets into what seems their dog's view, guiding the player through the first area of the game. The area where the homeless man resides becomes the flood gate's opening, And entering that area will cause the vision to go away, and go back to the player's point of view. After that, behind the flood gate there will be a loud screech heard, and at this point the player has to activate the flood gate to make the Siren go away. If the gate isn't flooded, the siren will destroy the door and kill the player.
Area 3, The Great Old Beast in the Great Old Wood
After killing The Talking Brain, The Player will be ambushed by The Siren and the path back out will be blocked. The player must defeat The Siren by shutting a large door right on it, and attacking The Siren in order to proceed. Then, Childhood Memories will pop out of it's face, dying in only one shot. However you can leave the area without having to kill Childhood Memories.
After you leave this area if you reenter you'll be able to get a child's skull from the Siren which is a key to an ending in the game.