The Door
"The Door" is a diabolical door that holded Astral and the "Numbers" appearing in Yu-Gi-Oh! ZEXAL.
When Yuma's parents found the Emperor's Key in some ruins, they sent it to Yuma, who always wears it as it reminds him of his parents. Shortly after that the door started haunting Yuma Tsukumo 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.
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, as for Astral, he was 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 meet Astral. The door hasn't appeared ever since, but it might be explained why it holded Astral and why the Emperor's key brought Yuma to it later on the series.
