It is not a quest, just one of the many secrets that the Dungeon has/will have if you choose to explore.
The green orb is in one of the floors with green lighting, you need to defeat a mini boss to access the chest. I think it was the sixth floor, and you need to defeat the Goblin King, but my memory sucks.
I do not remember how many spells the game has currently, but a good part of them will be removed from the magic shop at some point and replaced with spells that require chanting.
There is no Riveria route, it was planned at the beginning, but scrapped once I realized I'd need to split the game in two. Riveria route will happen in the sequel (You can import a no-route save in there, but it is not required, just start a new game with no route there).