The Oblivion War is a secret war to banish the Old Ones to Oblivion. By erasing mortal awareness of the Old Ones, they will no longer be anchored to the mortal world and will be locked away in Oblivion. It is for that reason the Venatori struggle to keep the War a secret. There are also groups -like the Stygian Sisterhood - who want the Old Ones there as a route to power. This war has been going on for the duration of mankind.
The Archive's involvement Edit
As posted by the author on his online Forums:
I'm pretty sure this will never make it into the actual Dresden Files, since Harry has no idea the Oblivion War is happening, along with everyone else. So I'll share it here. :)
The Archive was constructed for the Oblivion War. Specifically.
Yes, the Archive (and Ivy, the two aren't really divisible) know about these forgotten beings. The Archive is in essence the keeper of the dead, where they are concerned. Once the archive believes one of them has been consigned to oblivion, she holds on to the memory of that being briefly, for another thousand years or so, watching for any mention of that being in print in an effort to make sure that she is the /last/ person alive who remembers whichever hideous entity has been consigned.
And once the safety period has elapsed, and the Archive is confident that no one else remembers, she deletes the memory from the Archive. Bad guy, /gone/.
She also tries to keep track of the enemy players in the Oblivion War via watching for communications and so on. When she finds a trace of them, somewhere, she lets a cell of operatives (like Lara and Thomas) know what's up, through a blind drop, and sends them off to handle the problem.
The Oblivion War is a huge, slow thing. Stuff happens every few decades, at most. That's why the Archive was created--to be an immortal awareness, something that could track and intelligently direct responses to the enemy in a war happening on an almost geological scale.
All that other stuff she says the Archive is for? Smoke and mirrors. :)