The Cathedral (Sinnoh)

The Cathedral is a building in Sinnoh located north of Stark Mountain (past the in-game Sinnoh map), built atop an Aura Nexus at an unknown date. Annoski was originally sealed within the Keystone by the Aura Guardians there, and it was where he met his final defeat. Containing architecture and artwork from Kanjoh, Sinnoh, and Almia, and described by Annoski as a place where "only those with a strong will and a willingness to bet their own lives" can find due to magical protection obscuring it from surveillance, it remains an enigma.

Trivia

• The Cathedral is located so far outside of civilization that large amounts of the local Pokemon have never seen a human before, and therefore do not follow the Vow.
• Its design is based upon the Lost Cathedral from Soul Calibur III.
• The Cathedral technically isn't a cathedral, but Tagg gave it that name out of an inability to figure out anything else.