The Oversoul is the Ultimate Plot Narrative Control device; possession of the OS allows the user to quite literally accomplish any action they wish. Like a normal Soul, it has the power to do the unimaginable; only without limits. So powerful it requires a prison to be constructed around it.
The power of the OS attracts many people to go into the Warrens, but they all meet the same fate.
The Warrens themselves are located around(Or on, depending if the Warrens
is) the OS. This is because the King of Haria trapped Oric and his Siblings inside the Warrens, as their punishments against the King. He might have thought it would kill(or drain) them, like every other person who enter the dungeon; but instead it didn't for whatever unseen reason.
Oric and his Siblings work to keep the OS under control, and by doing so have access to it's immense database on every single soul it encounters.
(Scanner connecting to the OS Soul Information Database)
The OS is intricately connected to every single Soul inside the Warrens, and a connection to the OS usually means certain Soul Depravity.
(The OS connection to an activated Soul Star; which constantly become deactivated)
The OS is a dangerous threat to the Warrens, which are built on the foundation which keeps the OS trapped within it's prison.The Void itself is a tool used to seperate the dangers of the OS from the Warrens, via preventing escape and offering a safeguard for the Sub-DLs. The OS itself is sentient, and wishes to escape it's prison; but to do so, it must bind itself to a life, which it already has:
CIIIIIIIIRRRRROOOOOOO IIIII AAAAMMMMM FFFFRRRRREEEEEEEEEEEE
The OS wishes to use Ciro to escape it's prison, but to do that it must destroy Oric. And when the Power went out in the lower sections of the Warrens, the OS guardians attempted to destroy the "intruder"
The area looks around for you.
Even the Angels themselves might be some of the Original Guards of the OS, and wish to overthrow Oric because of this. Maybe even reviving Riltia using their powers over the OS.