Pitioss is a bonus dungeon, as well as the most unique location in the entire game. You won't find a single enemy inside of it, as the whole dungeon is about exploration and it's filled with numerous arcade elements, requiring mastery of character movement.
However, before you can go to the dungeon you must first upgrade Regalia - more on that can be found in "Cindy - Upgrading Regalia" chapter. Afterwards, you have to travel north from Verinas Mart - Ravatogh. You can reach it only with the flying Regalia and the entrance to the dungeon can be seen on the above screenshot. Additionally, the entrance is accessible only during the night - it will be closed during the daily hours.
Describing this vast location is basically impossible - the area lacks any distinctive waypoints. Below you can find a video presenting the process of reaching the dungeon, as well as collecting the main treasure hidden inside - Black Hood.