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
Giratina randomises this cave's space and time. To get to it, 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 Pokémon increase in level every time you pass a pillar. However, in the entrance and rooms with a pillar, no wild Pokémon 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 there 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 Pokémon other than a waiting Giratina.

Battling
Giratina is an extremely strong legendary Pokémon but after it is captured. Right now, its 4 moves make the battle easy (so no need to defeat it). Here's how.
 * First reduce most of its power using a ghost, dragon, dark or ice type move from a Pokémon below level 70.
 * Paralysing it is also suggested. (Not burn, poison or confuse)
 * Once its HP is in the red zone, switch to a ghost Pokémon (makes the move slash useless). A flying ghost or one with the ability to levitate overcomes move Earth Power.
 * If these needs are met but you don't use a ghost, lots of healing items are needed. Heal Block move stops healing, so switch a blocked Pokémon to remove the block.
 * Giratina vanishes one move before using shadow force. Quickly switch to a normal Pokémon (immune to move Shadow Force).
 * Start throwing Ultra Balls or Dusk Balls if your Pokémon's HP is high and Giratina's is low. Try hard till you get Giratina

After battling
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.