Underwater Palace

Arthur jumped off the descending root and rolled on the ground below. The rift was closing after it had allowed the island, and he witnessed the site from the land below. 

After everything settled, the eerie silence of the surrounding struck Arthur harder than he expected. It seems he was alone again after meeting an old friend and a bizarre spirit. 

As for what Adrian told him before he left, Arthur could only make out a few things. First, a being, a human, or a spirit had tasked Adrian with building a city for the people to survive when the end arrives. 

As for the end itself, it was no other than what Jizo said will happen if humans kept going beyond the gate. Then, the split will undo itself, and the calamity gates will open. At that time, neither Alka nor Earth will survive.