The Door

"The Door" is a diabolical door that holded Astral and the Numbers appearing in Yu-Gi-Oh! ZEXAL.

The door was haunting Yuma Tsukumo in his dreams for some time 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, but will lose what they appreciate the most.", but Yuma always wakes up before he can open it.

During Yuma's duel with Ryoga Kamishiro, The Emperor's Key, which was broken at that time, responded to Yuma and took him to the door once again, repairing itself in the process. The door once again insisted 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 convinced him of the opposite, and shortly after that he meet Astral.

Despite the door's warning, Yuma hasn't lost "what he appreciates the most" yet, but he might later on the series. It is unknown as to what this object/person could be, he claimed it was the Emperor's key, but that might be wrong.