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Ninja Mansion (忍者屋敷) is a room presumably located on the 9th Floor of the Great Tomb of Nazarick.
The room was created by Garnet for Nishikienrai to serve as his personal playground while acting as a ninja. It was also the location for the guild's "Ainz Ooal Gown High School Trip".
Layout & Features
A Japanese-style room that includes sliding screen doors. The room was suppose to allow one to feel like a ninja. In addition to that, there are several gimmicks like a ceiling door and a revolving hidden wall.
There are apparently many ninja mansions located in Nazarick made by Garnet.