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.
The jar was found by Nishikienrai in a closet of the Ninja Mansion when he searched for place to hide from Luci★Fer. Tabula Smaragdina explained what it was, a vessel containing an evil spirit, which made the ninja all more careful not to knock it over by accident.
A large jar with multiple ofudas.
No abilities have been clearly stated on what the jar can actually do.
It is unclear whether the item was a decoration or an actual item. But given its appearance in the Ninja Mansion, the Demon Seal Jar is most likely part of a gimmick or trap created by Garnet.