The Millennium Key, known as the Millennium Ankh (Japanese:
It is in the shape of an ankh, the Egyptian symbol of life (thus leading some to refer to it as the Millennium Ankh), and is the only Millennium Item without an Eye of Wdjat anywhere on it.
In Ancient Egypt, Shimon used the Millenium Key to seal Exodia the Forbidden One into five stone tablets. During the battle with Zorc, Shimon was forced to use the Key to release Exodia. The Millenium Key allowed Shimon to control Exodia, but it also forced him to feel all of Exodia's injuries, which eventually lead to his death (or being banished to the Shadow Realm in the dub).
The Key, along with the Ring, Eye, and Scale, were guarded by a faction of the Tomb Keepers. The Ring and Eye were given to other people, leaving the Scale and Key to Shadi. After the Battle Ritual, the Key, along with all the other Millennium Items, were buried underground.
Its primary unique ability is the power to unlock the doors of a person's soul, giving the user access to the "Room of the Soul," where their thoughts can be observed, or the "room" can even be "redecorated", allowing the user to alter a person's personality. The brainwashing process can be undone, when the victim touches the Key. Also, the controller can manipulate and destroy the personality.
Additionally, it can allow the user make himself/herself (and others) invisible to physical sight, and allows them to sense any disruption of the Millennium Items' mystical energies that would be caused by an item being taken from its rightful owner. The owner of this item may also force images, scenes, or visions into the minds of others, as well as communicate telepathically with others.
This card is defeated in battle, but you can look at your opponent's hand and choose the next 2 cards that they will play and use to battle.