Turnback Cave is only available after getting the National Dex and is located in Spring Path (the fourth and secret lake of Sinnoh) on Route 214. The legendary Ghost/Dragon type Pokemon Giratina can be found here. Turnback Cave is actually a randomly generated maze consisting of square-shaped rooms with four doors, one on each side, along with some rocks. Whenever you enter a room, it is practically impossible to turn back to it (hence the name). There are special rooms with pillars where the goal is to pass through three of these pillars located randomly every time you enter into another room, before entering into a total of thirty rooms. If you are lucky enough to pass through three of these pillars, you will find a room with a huge pillar and a wild level 70 Giratina standing in the center. Once you catch/defeat it, take any door from the room to go the main room, in which you have the option of leaving the cave through the door at the south in front of the stone tablet, or to try to generate the map again to get to the room in which Giratina was found. Giratina will not appear again, but there are rare items you can get from the room. Depending on the criteria, you will get a certain item or event:
CRITERIA - ITEM Cross through three pillar rooms and not any other room - Reaper's Cloth (used to evolve Dusclops into Dusknoir). Cross through three pillar rooms and any other room - Rare Bone (can be sold in the Pokemart for as much as a Nugget). Cross through thirty rooms in total without achieving to cross through three pillar rooms - No Item; get to the starting room.