I have no idea what an "oubliette" is, bit it sounds rather naughty. :hehe:
An Oubliette is a kind of dungeon (as in prison cell) that could only be accessed from a hatch from above. The idea is to keep the prisoner from escaping by any means, as the only way out would be to go up with a ladder/rope, neither of which are accessible (except when lowering down food and water).
Watched the video, looks awesome, Dugneons of Ivellon and Oubliette are new to me, but looking forward to The Deep. Currently playing Adash which is wonderful~