Whoever can open this door will gain a great new power. But know that such a reward comes great risk. You would lose that which is most valuable to you.— The Door stipulating the conditions for those who open it.
The Door (扉 Tobira) is a diabolical gateway that appears in the Yu-Gi-Oh! ZEXAL series. There are two doors - one of which held Astral and the "Numbers" and is unlocked by the Emperor's Key, and one which held "CXyz" which is unlocked with the Barian Emblem. The Door is voiced by Inoue Kazuhiko (Japanese) and Marc Thompson (English dub).
When Kazuma Tsukumo obtained the Emperor's Key, he gave it to his son, Yuma Tsukumo, who always wore it as it reminded him of his parents. Shortly afterwards, the door started haunting Yuma in his dreams, causing him to have the same nightmare almost every night. In these dreams, Yuma meets the door and a voice says; "Whoever opens this door will obtain a new power, but in exchange will lose what they appreciate the most," but Yuma always wakes up before he can open it. The dub replaces the second half of the statement with "but know that with such a reward comes great risk". The voice then questions if Yuma has the courage to release the ability to discern the humans who are possessed by the "Numbers".
During Yuma's Duel against Shark, the Emperor's Key, which was broken at that time, responded to Yuma and took him to the door again, repairing itself in the process. The door once again told Yuma to open it, which he did using the Emperor's Key. When it was opened, Astral was released, but some shock caused Astral's memories to break into 100 blank cards. Astral was then fused with Yuma. While Yuma was being combined with Astral, he saw the one-hundred blank "Numbers" around him in a sphere. The sphere quickly broke and the "Numbers" were all scattered except for one, which Astral kept. When Yuma returned to reality, he thought it was just a dream, but seeing the Emperor's Key repaired convinced him otherwise. Shortly afterwards, he meets Astral.
At a later point, Yuma and Astral mentioned The Door when discussing the Emperor's Key.
Later, it appears once again in front of Yuma and reminded him of their covenant: The Door gave him a new power, but in exchange Yuma will forsake his most precious belonging. Then it says that someone is interfering with the deal, and gives Yuma the power of ZEXAL so that he can defeat Kite. It was revealed that beyond the other side of the door is an entrance to the dimension that is inside the Emperor's Key.
Sometime after the World Duel Carnival, Yuma dreamed about the door once again, and it repeated the same message. However, this time the voice told him that he will be losing that which is important to him soon.
After the duel against Girag, Astral tries to gather memories of the "Numbers" he had already collected, but still couldn't reach. Then he is transported to the door, who tells Astral it was going to test him, in order to obtain the true memories of the "Numbers", and says that at that time, everything would start to move. The door then vanished, and the Shadow Giant appeared in front of Astral, in order to test him through a Duel. After Yuma, who is brought to inside the Key, and Astral defeat the Shadow Giant, the door appears again in front of them, with Yuma recognizing that the door from his dreams. The door then tells Astral to remember his mission, showing him and Yuma a vision, which makes Astral remember the Numeron Code.
When Yuma Dueled against Nash, Yuma realized that with the power that he obtained with Astral, the trade-off would be to lose who he truly was, his idealistic ability to connect with others, which he ultimately defied. During his Duel with Astral, the latter used "The Door of Destiny" card, which transported him and Yuma in front of it. Yuma reflected how this was where he and Astral first met. Recalling the Door's words, Yuma came to realize what he will have to lose his most important thing - Astral. Although conflicted, Yuma had come to accept this at Astral's urging.
After Mirai Tsukumo gave the key to Yuma, he had dreams about it for years. The Door says that the one who opens this door will receive great power, but as compensation will lose something important to them. During his Duel with Shark, Yuma finally opened the door, leading him to release the "Numbers" and meet Astral.
Five years before the events of World Duel Carnival, Dr. Faker conducted research about finding the doorway to the other dimension, assisted by Byron Arclight. With Kazuma Tsukumo's help, they located the remaining two of twenty-three odd phenomenons in the world, which allowed them to deduce the location of the door. After finding the door, Faker betrayed both Byron and Kazuma, sacrificing them to enter the Barian World.
On the other side of the door, Faker encountered the Barian Emperor Vector, who agreed to heal Faker's son, Hart Tenjo in exchange for Faker destroying the Astral World. Vector stabilized Hart's life with his powers as Faker worked to complete the Sphere Field Cannon and collect the "Numbers". If Faker did not succeed, Vector would take Hart for himself.
When a duelist performs a Chaos Xyz Evolution by Rank-Up with their "Numbers" using a Barian World's "Rank-Up-Magic" card, the Barian Gate appears, with their "Chaos Numbers" coming from inside it. The monsters are of Barian origin, and belong to (because of the energy inside the monsters) or are from the Barian World.
- Yu-Gi-Oh! ZEXAL episode 1: "Go With the Flow, Part 1"
- Yu-Gi-Oh! ZEXAL episode 2: "Go With the Flow, Part 2"
- Yu-Gi-Oh! ZEXAL episode 19: "The Edge of the World"
- Yu-Gi-Oh! ZEXAL episode 24: "Hunting Down the Hunter, Part 2"
- Yu-Gi-Oh! ZEXAL episode 74: "Attack of the Barians: Part 1"
- Yu-Gi-Oh! ZEXAL episode 89: "Darkness Dawns"
- Yu-Gi-Oh! ZEXAL episode 143: "The Future of Three Worlds"
- Yu-Gi-Oh! ZEXAL episode 146: "Forever ZEXAL"
- Yu-Gi-Oh! ZEXAL Rank 1: "The Name's Yuma!!"
- Yu-Gi-Oh! ZEXAL Rank 2: "A Mysterious Life-Form?!"
- Yu-Gi-Oh! ZEXAL episode 55: "Portal of Doom"
- Yu-Gi-Oh! ZEXAL episode 70: "A Trio's Challenge: Part II"
- Yu-Gi-Oh! ZEXAL episode 83: "Sphere Cube Calamity: Part 2"
- Yu-Gi-Oh! ZEXAL episode 86: "Counter Offensive: Part 2"
- Yu-Gi-Oh! ZEXAL episode 88: "Dual Duel: Part 2"
- Yu-Gi-Oh! ZEXAL episode 98: "Shadows End"
- Yu-Gi-Oh! ZEXAL episode 102: "The Dark Mist Rises: Part 2"