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I present to you, my very (crappy) analysis of my very own Time Travel Sephiroth theory (with a poorly drawn timeline).
This is just theory by this point but I believe the Sephiroth that we saw in Chapter 18 is actually the Original Sephiroth who traveled back in time to reshape destiny. As a defense mechanism the planet summons the Whispers to make sure fate stays on track. The Whispers are confirmed to be connected to the threads of space and time. This explains the visions of the future that the party and other people see, because Sephiroth time traveling altered the threads that the Whispers are connected to.