Great Door

"Sing more!, Sing only for me!"

- Bourbon

The Great Door refers to a hostile supernatural phenomenon found at the beginning of Lost Catacombs.

Overview
The Great Door is a dark corridor with a great stone door and red light directly behind it. The room in the real world appears to be a large utility drainage room ominously littered with unscavenged corpses, indicating that many had fallen victim to it. Whilst in the room, Artyom is subjected to periodic visions of a black hallway and the titular Great Door opening opposite to him and Bourbon. After the first vision, gas pours into the room, which either chemically or psychically influences both Artyom and Bourbon, who succumbs to a blissful madness in which he fantasizes of the Great Door singing and calling to him. Upon Artyom's third vision of the Great Door, it's fully opened, and a vacuum attempts to suck both him and Bourbon inside, not ending until Artyom makes his way toward the light. Not wanting to leave the paralyzed Bourbon behind, he tries to drag him to safety. Unable to, Artyom reaches out to the light in desperation. A Dark One grabs his hand and drags them out of the vision, saving them both. Once on the other side of the metal grill and away from the dangers of the drainage room, Bourbon recovers and asks Artyom if he heard the songs too, before stating that he would not wish it even on an enemy.

Trivia

 * The Great Door may literally be a door to the afterlife, as Khan later mentions that Heaven and Hell were atomized after the war.
 * The term "Great Door" was coined from one of Bourbon's quotes: "Oh great door, I hear you are calling me!"
 * In Metro: Last Light, the Baby Dark One mentions hearing a Great Door calling for Artyom. This might be in reference to the Great Door in the first game.
 * It's interesting to note that the Great Door affects only humans as the Dark One who saved them was not only immune to it, but dragged them both out of the vision. A group of Nosalis also managed to chase after them through the room after this vision.
 * Its hypnotic effects are somewhat similar to the Red Star shown in the novel Metro 2033. This might be an easter egg or direct acknowledgement from the game.

Listen to that quote in English: