If an illithid hero attempts to use their psionic abilities on a beholder, it fails and they get confused instead, because the beholder’s mind is so alien that they can’t comprehend it.

A monster passive ability (possibly for shambling horrors or beholders) that unmaps its surrounding. This would be most useful when the monster also has something that incentivizes blinding yourself when near it, such as gaze attacks.

However, this can be negated by player note-taking of the map before the monster wipes it, so care should be taken not to make that worth doing.