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 dream 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 the dream always ends before Yuma can do anything.

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 combined with Astral, he saw the 100 blank Numbers around him in a sphere, but the sphere broke and the Numbers where scattered away except for one which Astral kept.

Despite the door's warning, Yuma hasn't lost "what he appreciates the most" yet, but he might later on the series.