The Door

"The Door" is a diabolical door that appears in the Yu-Gi-Oh! ZEXAL series. There are two doors, one of which held Astral and the "Numbers", unlocked by Emperor's Key and one that held "Chaos Xyz" and is unlocked with the Barian Emblem. The Door is voiced by Inoue Kazuhiko.

Anime
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 asks if Yuma has the courage to unlock the power to see who is corrupted by the "Numbers".

During Yuma's Duel with 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.

Manga
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.

Barian's Door
Five years before the events of World Duel Carnival, Dr. Faker conducted research about finding the doorway to 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 pinpoint 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 emissary Vector, who agreed to heal Hart in exchange for Faker destroying the Astral World. Vector sustained 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 Misael, Alit, and Yuma performed a Chaos Xyz Evolution using their "Number" monsters, this door appeared in the process, with their "Chaos Numbers" coming from inside it.