Giratina is one of the most powerful Pokémon trainers she can catch in Pokémon Brilliant Diamond and Brilliant Pearl. The legendary Ghost / Dragon-type Pokémon was the mascot of Pokémon Platinum and is a strong addition to any team in Pokémon Brilliant Diamond and Shining Pearl. That said, Giratina has two forms, and here's how to change it in the new Pokémon games.
How to change the shape of Giratina
Trainers who manage to catch Giratina will notice that the Pokémon is in its Altered Form. To change it, you simply need to give Giratina Griseous Orb, which will allow the Pokémon to transform into its Home Form.
It is worth noting that changing Giratina's shape will not affect its moveset. Additionally, Giratina will not be able to transform if she is holding any other items besides Griseous Orb.
Giratina can only be caught in Pokémon Shining Diamond and Shining Pearl in the Cave of Return. However, access to National Dex is required, which can only be obtained by beating Elite 4 at least once. Once you have access to National Dex, go to Turnback Cave near Sendoff Spring. Giratina is found in the room following the 3rd pillar; however, the Turnback Cave layout is randomly generated so it will take some RNG luck.