Ouroboros

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

Description
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, a guild used Ouroboros to remove Ainz Ooal Gown's presence from the World itself robbing the guild of its mines.

Appearance
The World Item is said to be a ring.

Abilities
This World Item has the ability to banish a guild from any World. It erects a seal around the World for a undefined amount of time, preventing access. Only those that possess a World Item are immune to its restrictions.

Trivia

 * Ainz Ooal Gown is particularly wary of this item, as it represents the possibility of Nazarick's defeat.
 * The name Ouroboros refers to an ancient symbol depicting a serpent or dragon eating its own tail, which represents cyclicality.