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The Tanoby Chambers is a location in the Sevii Islands where you can find Unown once you've been through Tanoby Key. Certain Unown are in certain chambers. There is a pattern in every chamber. The pattern is how the walls are shaped, which is different in every chamber. As you get enough Unowns you can get another Tanoby Chamber. In this Tanoby Chamber you can get the A-form, the ?-form, and the !-form. These ruins could possibly have a link with the Johto region's Ruins of Alph.
One theory we could make is that people from the Kanto/Johto mainland migrated south to the Sevii Islands. Note that this trek would've probably taken months and there would be vigorous storms and torrid weather as they travel towards the tropics. Once they finally reach Quest Island specifically, they make an altar to thank their gods or deities for letting them cross the oceans alive. Theory #2: There were already people there and they might've had some contact with the Unown or a Johto legendary such as Entei or Lugia. They build temples to house them. Theory #3: The Ruins of Alph could've been part of a city from an ancient civilization such as Pokemopolis and there was a massive raid. Most of the city is destroyed and only a handful of people survive. They make new chambers at Quest Island as a replacement for the destroyed portions of the city.