So we've got Heatran and Regigigas down, but there's still one more legendary lurking out there - and this one's perhaps the most fearsome of them all. As usual, you'll want to have a healthy supply of healing items and high-level Pokeballs before taking this sidequest on. You'll also need a pokemon with Rock Climb to reach the entrance to Giratina's lair. We also recommend bringing along a high-level Honchkrow - we'll explain why in a bit.
If you walk along Route 214, you might notice that a path has opened in the woods to the east. If you're wondering why you never saw this before, it's because it has just now opened up to you. When you enter, you'll be greeted by a huge, grassy field, along with some forbodingly creepy music. Trudge through the grass and scale the hill using Rock Climb to find a cave entrance partially surrounded by a small lake. There's nothing to see in the water, so it's best to just head straight into the cave. (You may want to save your game first, since you can't use Dig or Escape Ropes once you've gone inside.)
We're now inside Turnback Cave, which looks less like a cave and more like a temple of some sort. Foreshadowing! Upon entry, you'll be greeted by a huge cloud of fog, as well as a riddle describing how to find Giratina. (You can use Fog Clear if you want, but it's not necessary.) In order to find Giratina, you must find 3 pillars within 30 rooms. After you reach the third pillar, the next room you'll enter will contain Giratina. After each pillar you find, the wild Pokemon you encounter here will become 10 levels stronger on average, going all the way up to (yikes!) the high Level 60s.
Now, you can either stumble around in the fog like a moron looking for pillars, getting trashed by incredibly strong wild pokemon along the way... or you can just follow the incredibly simple route we'll show you in the movie below.
Giratina waits for you in the center of its room. Like the other rooms in Turnback Cave, it's quite foggy in here - but we actually recommend you leave the fog intact rather than using Fog Clear. Why? Because while fog impedes your accuracy, it also hinders Giratina's attacking ability as well - and the more attacks it whiffs, the better. (If you opt to leave the fog up, you may want to use an accuracy-boosting battle or hold item.)
As for Giratina itself, it's a unique Ghost/Dragon type combination, meaning that it's vulnerable to Ghost, Dragon, Ice, and Dark techniques. Fighting and Normal attacks are completely useless against it, and Water, Poison, Bug, Electric, Grass, and Fire skills are not very effective. It has three damage-dealing attacks: Slash, a normal-type physical strike, Earth Power, a Ground attack that can lower Special Defense, and Shadow Force, a Ghost-type skill that leaves Giratina invulnerable for one turn (much like Fly or Dig). It also has Heal Block, which prevents a Pokemon from recovering HP for 5 turns.
Remember how we recommended bringing a Honchkrow along? That's because Honchkrow is the perfect Pokemon to use for keeping Giratina at bay. Honchkrow is a Dark/Flying type, meaning that Giratina's Shadow Force will only deal half damage against it - and Earth Power will have no effect at all. Honchkrow also has above-average Defense and HP, which will also help it withstand damage from Slash and Shadow Force. Its Dark- and Ghost-typed attacks can help get Giratina's HP low, while a good Flying attack like Aerial Ace can whittle it down even further. The only real threat is Heal Block, which can be dangerous if it takes effect when Honchkrow's HP is getting low. If need be, use Battle Items to up its Defense to further reduce the damage taken from Giratina's physical strikes.
Once Giratina's HP is low enough, you can start throwing Pokeballs. As with the other Legendaries, you'll want to make use of Hyper, Timer, and Dusk Balls for the best chance of securing a catch. Make sure you've got a good defender (like the aforementioned Honchkrow) out to absorb Giratina's attacks while you keep on trying to capture it.
Congratulations, the almighty Giratina is now under your beck and call! You've tamed the most fearsome beast to ever cross into Sinnoh... for now, anyway.
So now we've got the three Level 70 legendaries. What else is there to do?