Even after Completing the Game and Unlocking the Dungeon to acquired the Legendary weapons the World of FF XV offers so much more to the player, When completing this side quest you will gain accesses to much Harder dungeons. Final Fantasy XV Stealing The Past Side Quest Guide
Stealing the Past
According to IGN, the “Stealing the Past” side quest can only be started once you finish the game and have unlocked the Regalia Type-F. In Chapter 15, head to Lestallum and go to the eastern part of town at night or on or past 21h.
Locate the window in the image above. Noctis will then eavesdrop on two thieves. Now, head to Verinas Mart – Ravatogh and talk to the tipster here. They will reveal the location of a treasure near the Rock of Ravatogh. Head to the new quest marker on your map.
Try to fly the Regalia Type-F to the entrance to the Pitioss Ruins in the morning so you can see the landing strip clearly. Make sure you land the Regalia precisely because if you miss the landing strip you can crash, which will result in a “Game Over.” Completing “Stealing the Past” rewards players with 50,000 EXP and a Magitek Core, which can be used to upgrade the Drillbreaker.
Samurai Gamers has revealed that the door to the Pitioss Ruins is only open at night, so you can wait around until the door opens by fighting some enemies near the entrance. The Pitios Ruins is a “constantly moving dungeon” that takes some time to get used to. Players must make every move count because finishing the dungeon will require a lot of patience, “accuracy, timing, and luck.”
Watch another Video on YouTube to see whee all the treasures are in the Pitioss Ruins, When finishing the dungeon yu will acquired the Black Hood which allows Noctis to automatically evade all attacks when equipped. Final Fantasy XV Stealing The Past Side Quest Guide