Missing Ring is Antinomy's Duel Disk.

This Duel Disk has not been seen in the anime. It has only appeared in the video games Yu-Gi-Oh! 5D's Tag Force 5, Yu-Gi-Oh! 5D's Tag Force 6 and Yu-Gi-Oh! 5D's World Championship 2011: Over the Nexus

In Yu-Gi-Oh! 5D's World Championship 2011: Over the Nexus, the player can find Missing Ring in a chest in the Duel Monsters Spirit World.