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This is the best explanation...in comparison, Ventus=Cloud and Van=SephirothThe first one is Ven taken over by Vanitas before he fights Aqua.
The second one is Ven surrounded by Unversed that Vanitas summoned. This is before they fight.
The third one is apparently Ven's Awakening pillar. But Vanitas and Ven have their final fight here. Vanitas has the X-Blade during this battle. After Vanitas is destroyed the side that has him on it disappears.