This door is found in the lounge of the underground shelter. The players initially believe that there are three doors in the shelter, one in each ward. However, it is eventually revealed that the three wards are the same ward, meaning there is only one X-Door.
In order to open it, six X-Passes are needed. X-Passes are only gained by killing another person who is participating in the Decision Game. However, according to Zero, the door only opens for thirty seconds and then will never open again.
If the panel used to input the passwords is damaged, the only way to open the door is by using the quantum computer which Sean can access. It is unknown if the quantum computer could be used to open the X-Door again after it has been opened using the X-Passes.
Whether or not an escapee can open or break down the door after escaping is often left ambiguous, as Akane didn't break the door down in D-END: 2, while Carlos blew it up in C-END: 1. It is possible Zero allowed Carlos to blow open the door while preventing Akane from doing the same, as this was necessary to create two essential timelines - the one in which the AB Project takes place and the one in which Delta and Phi are born and transported.
Zero Time Dilemma
In numerous timelines of the game's story, this door is opened by numerous combinations of characters. However, not every ending results in the characters escaping through this door. That includes D-END 1, where Diana, Phi, and Sigma escape through the decontamination room. The door can also be opened through the Quantum Computer, as heard in Q-END: 2. Carlos blew the X-Door open during C-END: 1.
- The X-Door may be a reference to a "plague cross", back when plagues were common. These crosses were put on doors to signify if the occupants of a building were infected with a potentially contagious disease.
- The X on the X-Door is likely also a reference to there actually being ten participants in the Decision Game, since X is 10 in Roman numerals. This makes it the Decision Game's equivalent of the Number Nine Door.
- Originally, some people thought there were 6 X-Doors, corresponding to each X-Pass.