Hell I might as well give this a try.
Source is from various spoilers in the 2nd part (concerning the basis of the world and everything).
Spoiler for Massive Spoilers:
Let's start with
Shisai
Extraterrestrial (I think extra-dimensional might be more proper) beings. Lies outside of the "truth" of this world. Encompasses all darkness (?). Looks extremely unpleasant, looks nothing like god.
Left the world after creating the "first" Refal Edia (the oldest of the Edia ancestry) and Glouvil (also called Sword of Rebirth). Created the axioms of this world with the exception of the Lonely Demon, of whom not even the Shisai know when was created nor purpose of existence, hence they are cautious of this entity.
Ereru
Also lies outside the "truth" of this world. The origin of Shisai and Glouvil.
The End of the World: This is the murky part.
Goddess
The devourer of this world.
According to ancient legend that Shion have read regarding light and darkness, the goddesses lived for too long and their desires had became insatiable and went insane. They devoured their creations (light, darkness, and human) and was still not satisfied. They went on to devour the origin of all things, their own creators; thus, the world ended, and rebirth happened.
Gastark Legend
The Goddess put a barrier called human (ningen) on the Mad Hero, who was the doom to end the world, and sealed him to the south of the continent. The Mad Hero, even with all his power, could not break the barrier; thus, he ate the Weaver of All Equation, half of the Lonely Demon, who was also in the south to create human alpha (ningen α) to kill off all human to break the barrier. After breaking the barrier, the Mad Hero murdered all the Goddesses and devoured the "thing" the Goddesses were protecting. The power was beyond what the Mad Hero could handle, so he devoured the other half of the Lonely Demon, Solver of All Equation to control his darkness. At the end, the Mad Hero brought an end to this world as prophesied amid his crazed laughter, and rebirth happened.
Except the Lonely Demon. Because of "certain existence", the Lonely Demon cannot be reborn and is doom to eternal pain and suffering (which is the reason why Shion want to kill Ryner, to make sure this doesn't happen).
Feel free to rectify me if any of this is wrong.
There! You spoiler hungry people have something to think about in the mean time =).
I'll try to update other stuff when I have time.