Located in a ruined building exposed to the harsh elements of Nuva Island, the Gyth Seal is a Xana-infused device that prevents the demonic Gyth from entering our world. It's supposedly a one-way portal, but the Gyth were able to burst their way through for four hours in order to steal the Nuva Xana Crystal.
A similar seal is on Zeris Island, but this is a two-way portal; as the Gyth collect Xana Crystals, they are able to bring more of their kind through the portal on Zeris Island.
In the prologue, Richard and Charlotte chase the Gyth here, only to be encountered by Raul of Gyth, who obtains the Xana Crystal and heads back to the Gyth realm...only to return to Radia ten months later...