From A Wiki of Ice and Fire
The Gods Eye is a large lake in the Riverlands and marks its southern border. The great castle of Harrenhal and its town, Harrentown, sit astride its northern shore. At its center is a lone island, the Isle of Faces, the island where the pact between the children of the forest and the First Men was created. The water of the Gods Eye is green and warm. Among its feeder streams is the Rippledown Rill.
According to semi-canon sources, the Gods Eye feeds a river that in turn, feeds the Blackwater Rush.
During the Dance of the Dragons, the Battle by the Lakeshore was fought along the western shore of the Gods Eye, while the battle at the stony ridge occurred south of the lake. Princes Aemond and Daemon Targaryen and their dragons, Vhagar and Caraxes, fought to their mutual deaths above the Gods Eye.
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