Where is the Lustrous Orb in Platinum?

The Lustrous Orb is not an item in Pokemon Platinum. You may be thinking of the Lustrous Sphere, which is an item that is used to evolve Spiritomb into Gengar.

Here's how to obtain the Lustrous Sphere in Pokemon Platinum:

1. Beat the game: You need to have completed the main story of Pokemon Platinum to be able to get the Lustrous Sphere.

2. Go to the Trophy Garden: After the post-game, you can access the Trophy Garden.

3. Talk to the NPC: Talk to the NPC standing near the center of the garden.

4. Receive the Lustrous Sphere: The NPC will give you the Lustrous Sphere as a reward.

Remember, the Lustrous Sphere is used to evolve Spiritomb into Gengar by trading it while holding the sphere to another player's Pokemon.