This is a secret chamber that can be found by going into the fireplace in the Keep of the Lead Lord and unlocking the hatch. The Oubliette is where you'll need to go to find the Old Crest so that you can enter the Abbey of the True Gun. The key to this floor is avoiding poison and you'll be fine.

Should you go to the Oubliette?

Going to oubliette is worth it if you have the extra keys. It gets you more gold, chests, and a boss item. Basically more items (two chests and one boss item) and money - also it helped me quite a bit to get used to certain enemy types.

How do I get to the RNG department?

It can be accessed by giving the Sell Creep one Master Round and any other 2 items or guns (including Junk) in a special room in the Hollow. This room is always available if you enter The Hollow with at least one Master Round.