Emperor's Key

The Emperor's Key (Ō no Kagi), simply known as The Key in the English version, is a pendant belonging to Yuma Tsukumo.

The Emperor's Key is a mysterious artifact given to Yuma by his parents (mother in the manga, father in the anime). In the anime it is revealed that Yuma's father obtained it from someone similar if not Astral himself, and when that occurred Yuma's father saw the attack occurring to the Astral World.

The Key allows Yuma to bring Astral forth which is similar to that of when Yugi Muto turns into Yami Yugi. Astral can go into the Key and see the large structure seen in the first opening. There is a structure in the key that according to Astral is some sort of "spaceship". Inside the ship's construction is some sort of puzzle which seems to be made up of hundreds of gears marked with numbers that react with the Number cards that Astral absorbs making them move and glow. When a number appears on the machine it's starts turning on pieces of that mysterious machine. Astral assumed that it had to do with the person who is responsible for the loss of his memories.

At the beginning of the series a diabolical door started haunting Yuma in his dreams trying to convince him to open it. The Emperor's key was also the key to open that door, which released both Astral and the Numbers.

The Emperor's Key has power to release the "Number's" "alternate form", "Chaos Number" and make Astral visible.

As shown in episode 20, the Emperor's Key seems to give the user some sort of immunity against the Numbers' influence (or at least reduce it). This has been witnessed when Number 96 tried to take "Utopia" from Tetsuo by force and when Tetsuo Summoned "Utopia." Then again in episode 22 when Shark had the key and took control of Kaito's "Acid Golem". This is possibly due to its connection with Astral. This can also be supported by the fact that Yuma has not been influenced while others are almost "possessed" by the numbers and the number appearing on the user's body. The Number cards also appear to lock into the structure and become apart of it as seen in episode 22 which keeps them safe. Where they are printed on square pillers that stick out and glow. But if the key is spearated from Yuma's deck then the pillers go back into the structure and the "Numbers" become cards again meaning they are in danger of being taken. When it happened, some voice said that "Astral is immersed in the guidance of time" and that he must obtain the "Power of ZeXal". The voice described it as a power that turns all darkness into light. Intrestingly, that voice sounded excatly the same as the diabolical Door.