When the sun fell down on the planet, so did a ocean that acted like a barrier between two worlds. 
Asphid was some kind of shell covering Therasis and it was functional due to a single ancient dragon giving the energy to keep the sun ablaze. He was the bastion of all the worthy people there in the planet. Until One little being took away all of that energy. The dragon collapsed and died, and so did Asphid with him. But this little being was unable to bear the power of the sun, he imploded into a black hole, destroying Asphid, the shell, the oceand and the artificial sun it had, which rained as debris all over the planet. Few were saved from those days now long gone, but the life goes on in Therasis, keeping a giant radioactive crater as a scar of the things that happened. 

Art by me uwu