The Magmarizer is one of the several evolution items you’ll need to hunt down in Pokémon Brilliant Diamond and Shining Pearl. Namely, you’ll need this Pokémon to evolve Magmar into Magmortar. It’s the only Pokémon in these games that uses this evolution item, so you don’t need to seek out multiples of it. Here’s what you need to know about where to find the Magmarize in Pokémon Brilliant Diamond and Shining Pearl.
The best way to find this item is to locate it on a Pokémon, namely, Magby. Magby is the beginning evolution for Magmar, which means if you catch a Magby that you want to use, you’re going to have the evolution item already to reach Magmortar.
You can find Magby wandering around the Grand Underground. Players have noted that this Pokémon spawns shortly after they defeated the third Gym Leader, Maylene, but they also caught it before the fourth Gym Leader, Wake. Therefore, you only need to beat Maylene for it to spawn.
If you’re trying to capture a Magby in Pokémon Brilliant Diamond, unfortunately, Magby is a version exclusive Pokémon. You can only catch it in Pokémon Shining Pearl. If you have a friend or can find someone that can trade a Magby carrying a Magmarizer, then you’ll be able to receive this specific item. You can also receive it from the Battle Tower.