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If it's the one inside the Spire (purple area) it's probably not the Monolith, because as far as I can tell, the shadow can't be the Monolith. However, yes, it should be a form that you already have, that was required for you to get to that point. All you should have to do is approach the shadow with the right form and it'll open the path. My best guess is to try either the Mirror form, or the Egg form.
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Let me know if this helps!