"WHAT IS AN OUBLIETTE?
Defining a dread oubliette is quite simple. It is also terribly complex. In the plainest of terms, an oubliette is a space within the Mists with its own solid reality, but normally uninhabited except by those creatures who roam the Mists -- or the occasional misfortunate Mist-led or Fugued being. Such places are not necessarily corrupted, such as those horrifying sinkholes of evil. Although surrounded by the Mists, they are neither difficult to enter, nor usually to leave. They are places of sanctuary for some, holding cells for those unfortunates caught in a time fugue within the Mists or temporary homes for others." (chapter 8, p. 82)
"What makes the oubliette different from any of the other dark pockets in Ravenloft? Well, first of all, these places are not regularly inhabited by intelligent beings -- no villagers or no dread villain on the parapets of the castle. Yet these places have their own personalities and may be the temporary home of any Mist creature, whether born of the Mists, Lost or Mist-led and desperate to get out. Secondly, the oubliettes are never permanent..." (chapter 8, p. 85)
In Table 8-1, it is noted that an oubliette could have "salient effects," either at random or as determined by the DM. These can be "paranoia" (-2 penalty on attack, saves, skills and abilities -- Will negates), "twisted perceptions" (20% miss chances for all attacks, skill penalty of 5 -- Fortitude negates), "inside the nightmare" (lost sense of direction, tracking penalty of 5 -- Will negates), "raging heart" (rage effect on those in the oubliette), "menacing atmosphere" (-2 on all attacks, saves, ability checks and skills; paladins not immune -- Will negates), and lastly creatures who have used the oubliette for refuge for a week or longer gain spell resistance of 5 (stackable) until they have been away from it for a week.