The Life Orb is a held item that drains the life of the Pokémon holding it. This may sound bad, but it gives the benefit of increasing the Pokémon’s attack power. Each time the Pokémon lands a hit, the Life Orb drains some health. The health taken is not a huge amount which makes this item great. Here is where you can find the Life Orb in Pokémon Brilliant Diamond and Shining Pearl.
To get to the Life Orb, you first need to make it to Stark Mountain. To reach this area, you will first need to defeat the Elite Four and obtain the National Pokédex. After both of those objectives have been completed, head to Snowpoint City and take the boat over to the Fight Area. Head north on Route 225 to the Survival Area. From here, head east until you reach Route 227. You will know this is the right area because ash will fall from the sky.
Head north and stop at the house along the way if your Pokémon need healing. Take the stairs near the house and ride your bike along the platform to the other side. Head north through the tall grass and jump some barriers to reach the trainer on the right side. After defeating him, head north until you reach the stairs.
Instead of taking the stairs, use Rock Climb on the cliffside to the left. This will lead you up to a trainer. Defeat him and use Rock Climb on the cliff behind him. Go to the left when you reach the bottom of the cliff to find another area you can use Rock Climb. Take the rocks up and you will land next to an item. Pick it up to get the Life Orb.