You can unlock higher difficulty dungeons for Special Dungeons by unlocking the appropriate Normal Dungeon tier.
However, keep in mind the difficulty for Special Dungeons is not entirely relative to the difficulty of Normal Dungeons. <br>
(Typically for the Event Dungeons, they are much harder compared to the difficulty required to unlock them.)
[[:Category:Dungeons|List of Special Dungeons]]
Changes - Dragon Poker English Wiki