After the defeat of Malpercio atop Cor Hydrae, at least four (Mira is not shown) of the Five Great Nations, Wazn, and possibly other islands, began their descent back to the now unpolluted Earth. Sadal Suud, Diadem, Anuenue, and Alfard are gently guided down by the giant Sibling Gods, speculated to be a gesture from the five siblings who founded Malpercio. After Xelha releases the Ocean back onto Earth, Wazn soon follows suit coming to rest in the middle of the Ocean.
It's not known exactly which combination of the following factors caused the islands to fall. Many feel that it was a combination of all three.
Before Kalas and co could infiltrate Cor Hydrae, they needed to breech it's outer barrier. This was done by having the Great Nations use their End Magnus to fire beams of energy that deactivated Cor Hydrae's barrier. The End Magnus provided the power for the islands to float and much of their power had just be expended.
Malpercio, the being whose power was contained in the End Magnus, had finally been defeated; not just sealed as before. Some think that the defeat of the Gods (Wiseman 20 years prior, and now Malpercio) factored into ability of the islands to return to Earth.
State of the EarthEdit
Over the last 1000 years, since the War of The Gods polluted the Earth, the Taintclouds had been used to slowly cleanse the Earth, making it habitable once again by humans. That point had come, accelerated by Malpercio's defeat some speculate, by the time the Return to Earth took place. Some feel that the system the Ancient Wizards designed for the Nations to remain afloat would not have allowed a return to the polluted Earth.