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"As light shines from the northwest skies, from the tower's shadow an arrow flies. Pierce heaven's light to reveal the prize."
— Kass

Sign of the Shadow is a Shrine quest in The Legend of Zelda: Breath of the Wild. It is a quest given to Link by Kass, the Rito minstrel, atop Gerudo Tower. The quest is completed by firing an arrow towards the sun from an ancient platform directly southeast of the tower while the tower's shadow is over the ridge (earliest possible time for this is 15:00 in game). Once this is done, Sasa Kai Shrine will emerge from the ground near the platform. This may also be done at 1:00 by shooting an arrow towards the moon.

Spoiler warning: Spoilers end here.