"Oblivion" was supposed to be a "secret" level, but wound up on the cutting room floor during the years of development. It's been restored, somewhat, as a Quake (Anachronox' engine) map. You can find more information about the map, and the man who restored it here @Digital-Eel. The download link to get the Quake map is at the bottom of the page. There aren't any quests or anything, it's just a nice place to visit. Or play a quickie deathmatch in.


[Shameless plugging]
While you're there, check out the rest of Digital Eel, and bag a free copy of Strange Adventures in Infinite Space, which is now completely freeware.

That's a pretty neat map.. I wonder how it would have fit into Anachronox.

It probably didn't, and that's why it was dropped. Maybe they thought briefly about heading in a "put the characters in a mystic dimension, home to an all-seeing oracle that will reveal all major plot points right before the last major battle", and then figured it'd been done to death. Or, like the description says, it's a secret level, and upon arrival everyone would have been "WTF", and the quests involved would be nothing but comedy relief. Like slaughtering giant-fanged rabbits or something.

I guess this was from when they were still using the Quake 1 engine?

It's actually a holdover from when Romero and Hall started fiddling with the game's concept using custom Doom 2 maps. It's hard to believe that a piece of development material that had been in the planning process for so long got left behind.