Oubliette

From LanceaSanctum

For sensitive or dangerous cases, clerical inquisitions sometimes use special prisons, known as “oubliettes”. An accused that is sent to an “oubliette” disappears from the face of the earth oft times until the start of the actual trial. No one except the clerical inquisitors know where the defendant is locked up or whether or not he is still alive. Not even the Prince, a bishop or coterie-members of the defendant have the right to inform about the prisoner. The defendant is sent to an “oubliette”, for as long as the inquisitor-general who is investigating the case, deems necessary. - from "Inquisition in the Low Countries"-