Ouroboros

Ouroboros (ウロボロス) is one of the Twenty World Items of YGGDRASIL.

Background
In the past, Ainz Ooal Gown had laid claim to the Seven Hidden Mines. To prevent Ainz Ooal Gown from creating a monopoly on the rare metals within the mine, an enemy guild used the World Item known as Ouroboros to remove their presence from the World itself for at least a month. Once the duration effect of Ouroboros expired after one month, Ainz Ooal Gown reentered it and realized that the enemy guild had already seized the valuable metals within those mines.

Appearance
Ouroboros is said to be a ring.

Abilities
As a World Item, Ouroboros possesses a power which is said to be a superior version of the Super-Tier Magic, "Wish Upon A Star." For instance, it can directly make contact with the operating company who was responsible for the development of YGGDRASIL and request one wish the holder of Ouroboros wants granted from them.

In the past, a guild used it and requested the isolation of a certain world, preventing all players but their guild from entering for a month. In other words, it erected some sort of unknown seal around the world that stops players from gaining access to it. However, only those who possess a World Item are immune to its restrictions.

Trivia

 * The name Ouroboros refers to an ancient symbol depicting a serpent or dragon eating its own tail, which represents cyclicality.