Magmortar is the evolved form of Magmar and can be a great Fire-type Pokemon in Brilliant Diamond and Shining Pearl.
There are a couple of ways to get your hands on Magmortar in Pokemon Brilliant Diamond and Shining Pearl. Each of them requires trading with another trainer, however.
Magmar evolves into Magmortar when traded, or players can simply trade with another player to get a Magmortar outright. That is the case for Brilliant Diamond players, as the pre-evolutions are exclusive to Shining Pearl.
How to obtain Magmortar in Pokemon Brilliant Diamond and Shining Pearl
Catch Magby in Shining Pearl
Magby is the first form in Magmortar's evolutionary line. If you want Magmortar, you will need to get a Magby. That is, unless you straight up trade for a Magmortar or are on the receiving end of a Magmar and Magmarizer trade.
Magby can be found from the start of the game in the Grand Underground. Its biomes are the Volcanic Cave, Sandsear Cave, and Typhlo Cavern. You may have to catch a few.
The Magmarizer item can only be acquired from a wild Magby and not all of them are holding it. Catch a handful and hopefully the item is attached so you can start thinking about Magmortar.
Evolve Magby into Magmar
Evolving Magby is much simpler than evolving Magmar. You just need to get Magby to level 30 in your copy of Shining Pearl. Unless you traded for one to get it in Brilliant Diamond.
Battle with Magby in your Pokemon party and it will receive EXP any time a battle is completed and it hasn't fainted. You can also use Rare Candy to boost its level by one at any time.
Evolve Magmar into Magmortar
Now comes the point where you can evolve Magmar into Magmortar in Pokemon Brilliant Diamond and Shining Pearl. You have the Magmar and you have the required Magmarizer item.
Have Magmar hold the Magmarizer if it isn't already. Simply trade the Magmar to another player and when it arrives, it will evolve into Magmortar. Make sure you trust the recipient so you can get your Pokemon back.