Complex sentence: Upon learning about them from Bernie, Casey destroys the Dream Plug which not only wakes the Turtles but also frees the Dream Beavers who convert into a smaller size and return to their dimension as Bernie finally falls asleep.
Split into simpler sentences: Upon learning about them from Bernie, Casey destroys the Dream Plug which not only wakes the Turtles but also allows the Dream Beavers. It turned out that the Dream Beavers are actually small in size to everyone else causing them to return to their dimension as Bernie finally falls asleep.