Beating the Elite Four and becoming the next Pokemon champion has never been the only goal of Pokemon games - you'll also want to catch all the Pokemon that fill your Pokedex, and Shining Diamond and Shining Pearl are no different. You can normally find Pokemon all over the world or by trading version-exclusive Pokemon. However, some Pokemon require special methods to evolve or items to evolve. Magmortar is a great example of a Pokemon that needs a few extra steps to evolve, so let's see how it's done.
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Magmortar is the last of the Magby evolutionary line; as such, you will need a Magby, which is a bit difficult to catch. It is also a Shining Pearl-exclusive Pokemon, so Brilliant Diamond players will have to rely on trade to complete the Magby line. Your next step is to get a Magmarizer.
The Magmarizer, like Magby, is exclusive to Shining Pearl. Thankfully it's not too hard to find. Once you have the Magmarizer, give it to your Magmar as a held item. The next step in this process will require the help of a friend you trust.
Start a trade with your friend and send your Magmar while holding the Magmarizer and it will evolve into Magmortar. Now ask your friend to send Magmortar back to you. Try trading with a friend who is playing Brilliant Diamond so that both of you can fill out your Pokedex at the same time.