The Millennium Necklace (also known as the Millennium Tauk, pronounced "torque", in the Japanese, which is the specific type of necklace it is) is a Millennium Item.
was originally owned by the priestess, Isis, in Ancient Egypt, and became one of the two Items owned by the guardians of the Pharaoh's tomb throughout history, eventually coming to be owned by Marik's sister, Ishizu Ishtar, who is the reincarnation of the priestess Isis.
Using it to foresee her victory against Seto Kaiba in the Battle City tournament, Ishizu handed the necklace over to Yugi when her vision failed to come true, knowing that the necklace would be needed for Yugi to defeat Marik. After Joey became critically injured after his duel with Yami Marik, the necklace showed Yugi a vision of him and Joey dueling, giving him the courage to fight on.
"Ma'at" bears the necklace along with the other items.
The necklace allows its user to see through time, offering glimpses of the past and near future - futures whose outcomes can only be altered through the use of other Millennium Items.
It was later revealed that the necklace had protected Ishizu from the shadow magic of Marik's Millennium Rod, as Yami Marik had mentioned when he confronted Ishizu.