The Door
"The Door" is a diabolical door that held Astral and the "Numbers" appearing in Yu-Gi-Oh! ZEXAL.
When Kazuma Tsukumo obtained the Emperor's Key, he gave it to his son, who always wears it as it reminds him of his parents. Shortly after that the door started haunting Yuma in his dreams causing him to have the same nightmare almost every night. In that dream, Yuma meets the door and a voice says; "Whoever opens this door will obtain a new power, in exchange but will lose what they appreciate the most.", but Yuma always wakes up before he can open it. While in the English version, it said "but know that with such a reward comes great risk". Then asked Yuma, if he has the courage to unlock the power to see who's 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 100 blank Numbers around him in a sphere which quickly broke and the Numbers where scattered away except for one which Astral kept. When Yuma returned to reality, he thought that was just a dream, but looking at the Emperor's Key repaired convinced him of the opposite, and shortly after that he meets Astral.
The door was mentioned in episode 19 when Yuma and Astral were discussing about the Emperor's Key.
It appears once again in front of Yuma in episode 24. It reminds Yuma about their deal: The Door gave him a new power, but in exchange Yuma will lose his most precious belonging. Then it says that someone is interfering with the deal, and gives Yuma the ZEXAL power so that he can destroy Kite. It is also shown that at the other side of the door is an entrance to the same dimension Astral goes to when he enters The Emperor's Key.
