Pokémon Diamond and Pearl/Turnback Cave

Turnback Cave is a dungeon that opens with the Spring Path when you get the National Pokedex. Inside are high level Pokémon. Also, you cannot use an Escape Rope to get out. Before proceeding, use Defog to clear up everything and make looking around and battling much easier.

Giratina
To get to Giratina, you are supposed to enter three rooms each having a pillar in the middle, before going through total 30 rooms, excluding the entrance room. The key is to find an out-of-place rock in the formations, and go to a door near or opposite it. You will also need to use the Rock Smash move at times. Wild Pokemon increase in level every time you pass a pillar. However, in the entrance and rooms with a pillar, no wild Pokemon appear. If you exit any door by which you just entered, or fail to press A on three different pillars, you will be back at the entrance; then press A on the slab to restart the 30 rooms countdown. After seeing the third pillar, go through any door (except the entry) and you will be in the final chamber. Which has no wild Pokemon other than a waiting Giratina.

Note that whenever you come back after catching/defeating Giratina, after coming to the room where it once stood will get you an item, either a Reaper Cloth, a Rare Bone, or a Stardust (depending on how quickly you reach the room). The inscription in the entrance of the cave also changes after your battle with Giratina.