Spoiler Alert: WARNING! This page happens to contain a lot of heavy spoilers that are all originated from the Overlord Light Novel series written by the author Kugane Maruyama. To every Anime-only watchers and Manga-only reader out there viewing this warning, please mindfully consider your decision first before choosing to expose yourself on reading the information below. If you're not currently up-to-date with the main source material in the official books, do consider staying away from the page for non-spoiler sake until you actually read it first ahead of time.
Ninja Mansion (忍者屋敷) is a room presumably located on the 9th Floor of the Great Tomb of Nazarick.
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.