The Millennium Key' (also known as the Millennium Ankh) is a Millennium Item.
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.
After it was made the Millennium Key was given to Solomon Muto's Egyptian incarnate, Shimon, but later it was passed on to Shada. 3000 years later (5000 years in the dub), when Yugi solves the Millennium Puzzle, it's in the hands of Shadi, along with the Millennium Scale.
Its primary unique ability is its 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. The Millennium Key is the only item that does not have the Eye of Anubis.
Additionally, it can allow the user make himself, 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 and it gives the holder the ability to 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.