The Millennium Ring was the item held by Mahad in the past and Ryo Bakura in the present. Its powers are to guide the owner to where ever he seeks, acting as a sort of compass. 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. Its demonstrated abilities include sealing souls away, summoning Duel Monsters from the cards, and being able to divine the location of other Millennium Items. 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. It could only have this effect due to Zorc's evil powers.
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 took it back from them. Also, in a brief scene from the manga, Mana gained the Millennium Ring after the fight with Zorc.
Thousands of years later, 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 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 power 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, and 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, assumedly, Mana in the manga, and Seto Kaiba in the Yu-Gi-Oh! Monster Capsule GB video game.
The ring is also featured on Ma'at's chest.