This Pokemon is available after getting national pokedex. A new area will appear near veilstone. Go there go through the cave at the end of the lake (spring path) and there is a cave called Turnback cave with nothing but ghost Pokemon. There will be room after room after you go past the first two pillars and get to the third go to the room to the left and there will be Giratina you only have one chance of catching so save the game first before you fight.

Submitted by regigas385 (elmopeeps)