3,000 years before present day events (5,000 in the English anime), the Millennium Ring and the other Millennium Items were forged in Kul Elna by sacrificing 99 souls of the Kul Elna villagers. The items were created in order to protect Egypt from attackers. Mahad had the Millennium Ring before Thief King Bakura.
In the present day at the start of the anime Ryo Bakura had it, he later lost it to Yami Marik. After Yugi defeated Marik, Marik Ishtar gave the Ring to Yugi. When Rex and Weevil mistakenly took Yugi's Millennium Items in an attempt to steal his Egyptian God Cards, Yami Bakura retrieved it back from them. As the RPG replayed Atem's life, Thief Bakura managed to steal it again from Mahad. He once again gathered the Items and Summoned Zorc. Also, as seen in a brief scene from the manga, Mana gained the Millennium Ring after the final battle with Zorc.
Thousands of years before the events of Yu-Gi-Oh!, Alexander the Great came into possession with this item. During his time, he won many battles with it, and claims it to be his lucky charm, but years later, the Ring somehow corrupted his mind (possibly, it has the power to split a person's soul into two pieces: one is Alexander's light-soul and the other is Alexander's dark-soul due to the fragment of Zorc's soul and his power being sealed away).
In the manga, this ring has a gruesome effect on a wearer who has not the ability to maintain its shadow powers. If the wearer is not strong enough, then the ring will burn their soul and body, as seen when a thief who tried to steal it was forced to wear it by Shadi; after the ring's prongs punctured his chest, the thief had fire pouring out of his eyes and mouth and died horribly, a scene that Maximillion Pegasus saw, who was then subsequently forced to accept the Millennium Eye.
The strong people who managed to survive the ring's power are: Mahad, Ryo Bakura, Alexander the Great, Dark Marik, presumably, Mana in the manga, and Seto Kaiba in the Yu-Gi-Oh! Monster Capsule GB video game.
The ring is also featured on the chest of "Ma'at".
Its primary power is to guide the owner to whatever he seeks, including other Millennium Items, like a compass. It even detects non-physical objects like mystical energies or dark emotions. Like the Millennium Puzzle, it had a soul sealed inside of it, in this case a fragment of the soul of Zorc Necrophades (who had also lost his memories) and a portion of Bakura's past life, Thief King Bakura. Another well-known power of the Ring is its ability to seal one's soul, or portions of one's soul into other objects, including itself and other Millennium Items.
It can also control the magic of the Shadow Games. It can materialize Duel Monsters into the real world, initiate Shadow Games where the players cannot be disturbed, cast illusions, and manipulate souls. It also gives the owner dark powers, including erasing memories, clairvoyance, interfering with the powers of other Millennium Items, immobilizing people, and knocking people unconscious.
In the Memory World arc, it can put its owner's soul with last will into monsters like Mahad did to Illusion Magician, merging their ba with the ka of the monster. Bakura also used it to access his Diabound's ability to camouflage itself in the dark, so he could sneak into the Pharoah's palace, though he later states that Diabound becomes more powerful the more Millennium Items he holds (a trait likely true regarding most monster spirits).
Only in the anime, the Ring also has an unexplained bond with its owner, as shown when Bakura mysteriously regained the Ring after Tristan threw it off a cliff into the forest in Duelist Kingdom. This was presumably due to Zorc's malignant powers.
History of possession
|Holder||Means of acquisition|
|Mahad's predecessor||Made for Mahad's predecessor.|
|Thief Bakura||Stolen from Mahad.|
|Zorc Necrophades||Acquired after Bakura sacrificed himself to summon Zorc.|
|Alexander the Great||(Only in Yu-Gi-Oh! Capsule Monsters).|
|Mana||(Only in the manga)|
|Millennium Ring thief||Stolen.|
|Ryo Bakura's father||Bought the Ring while in a visit to Egypt.|
|Ryo Bakura/Yami Bakura||Ryou Bakura's father gave it to him, the Ring contained the evil spirit that became Yami Bakura.|
|Marik Ishtar/Yami Marik||Yami Bakura lost the Ring to Yami Marik in a Shadow Game.|
|Yugi Muto/Yami Yugi||After they defeated Marik, Marik Ishtar gave the Ring to them.|
|Rex and Weevil||When Rex and Weevil mistakenly took Yugi's Millennium Items in an attempt to steal his Egyptian God Cards.|
|Ryou Bakura/Yami Bakura||Yami Bakura retrieved it back from them (in the manga, he found it on the Battle Ship.)|
|Mahad||Used in the Shadow RPG.|
|Thief Bakura||Stolen from Mahad.|
|No one||Thief Bakura sacrificed himself and Diabound to summon Zorc Necrophades.|
|Seto Kaiba||(Only in Yu-Gi-Oh! MC).|
|Yugi Muto||RPG ended.|
|Ma'at||The ring is featured on the chest of Ma'at.|