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After the cave-in, Prisma tells the others she doesn't know what to do, since it wasn't part of her dream. No doubt Prisma, being the friendly and outgoing sprite she is, wouldn't even dream of one of her friends not wanting to come along on an adventure with her, which is why she was blindsided. The cave-in was caused by Bleak's wand falling because she wasn't there to catch it, but in Prisma's dream, Bleak was there, and the cave-in never happened.
In the Lite Sprites special, it's mentioned that dark shadows will take over Lite-Topia if Astra doesn't keep the stars lit (and, one assumes, if the other girls slack on their color-giving duties). However, a flashback shows Lite-Topia before the girls found their light wands, and not only is it not ruled by shadows, it's actually pretty colorful. This most likely means that when the girls removed the light wands from the crystal cave, they inadvertently unleashed Sealed Evil in a Can.