Here's how to get to it:
It's easy, first you have to acquire the lens of truth. Then look to the left side of the templar and you'll find platforms leading to a ledge far off. From there you need to play the Scarecrow Song and use the longshot to reach it!
[spoiler]It contains the hidden helmet: The Fierce Diety's Mask[/spoiler]
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The only tricky thing is it has to be done during the last day when the moon breaks the atmosphere and is heading towards Venus.