noun
oubliette
oo-blee-ET/ˌubliˈɛt/
A hidden dungeon accessible only through an opening above.
A trapdoor in the castle floor opens into an oubliette.

Etymology
French oubliette comes from oublier, “to forget.” Its grim image is a place where a prisoner could be forgotten: such dungeons held people condemned to perpetual imprisonment or to perish secretly.
In context
the prisoners would be thrown into the oubliette sometimes after intense torture and could possibly die in there
More examples
- The prisoner is lowered into the oubliette on a rope.
- Visitors peer down into the oubliette through its only opening.
- The guide points out two oubliettes beneath the castle floor.
Forms
oubliette · oubliettes