Though players do encounter a handful of Legendary Pokemon during the main story of Pokemon Brilliant Diamond & Shining Pearl, it's not until after they've completed the game that they'll be able to catch any of them besides their game's mascot. The Lake Guardians are perhaps the easiest to find in this regard, but if players do a little digging, they'll be able to track down a host of other great Legendaries, including the fire/steel-type Pokemon, Heatran.
Given the somewhat lackluster lineup of fire-types native to the Sinnoh region, Heatran provides a solid option for those who chose Turtwig or Piplup as their starter Pokemon. Of course, with players now able to get their hands on the likes of Charizard and Typhlosion via Briliant Diamond & Shining Pearl's Grand Underground area, this is nowhere near as useful as it once was in the original releases.
Where to Find Heatran in Brilliant Diamond & Shining Pearl
Heatran can be found in the depths of the post-game area, Stark Mountain, though before they can get there, players will first need to beat Cynthia and must also register all of the Pokemon that are native to the Sinnoh region in their Pokedex. Thankfully, the latter is nowhere near as difficult as it sounds, as the majority of players will have already encountered all but a handful of them as they make their way through the main story. For most players, it will therefore be a somewhat simple case of finding Drifloon (return to the Valley Windworks after defeating Team Galactic), Lumineon (evolves from Finneon at level 31), and either Palkia or Dialga (speak to Cynthia's grandmother in Celestic Town).
After seeing all 151 Sinnoh Pokemon (players will also need to hatch the Manaphy Egg that's obtained from the games' Mystery Gift mode), returning to Professor Rowan will lead to the player's Pokedex being upgraded to the National Dex. Once this happens, players should fly over to the Fight Area (or take the Ship from Snowpoint City if they've never been there before) and then head east onto Route 230. After surfing for a while, they'll be able to head north onto Route 228 and should then continue in this direction until they reach the entrance to Stark Mountain via Route 227.
Upon entering the volcano, players will be forced to team up with another trainer named Buck. They'll need to keep heading north until they reach a cavern containing an item called the Magma Stone, which Buck will take before leaving. After he does, players should head back outside and then head west onto Route 226. This will eventually lead them to the Survival Area, where Buck and his grandfather just happen to live. If players speak to the latter, who can be found in the house to the left of the Poke Mart, Buck will decide to return the Magma Stone. If players then go back to the cavern in the depths of Stark Mountain, they'll find a level 70 Heatran ready and waiting for them.
How to Catch Heatran in Brilliant Diamond & Shining Pearl
As always, players should save their game before interacting with Heatran just in case something goes wrong. Assuming they're not going to be using a Master Ball to catch the fire/steel-type Legendary, they should also make sure that they have a healthy supply of Ultra Balls in their bag, as Heatran can sometimes be a little tricky to catch. Once they're sure they are ready, players should approach Heatran and then press the A button to initiate a battle.
Given its high level, catching Heatran can be a little tougher than the likes of Uxie, Mesprit, and Azelf, as it is more than capable of dishing out a lot more damage. On the other hand, this does mean that it's a lot more difficult to accidentally knock Heatran out, so things aren't all bad. Even so, players should avoid attacking it with fighting, water, or ground-type attacks and instead opt for a type that Heatran has some resistance to (normal, psychic, ice, dragon, or fairy, to name but a few) just to be safe.
Should players accidentally knock Heatran out, they will be able to make it respawn by beating the Elite Four again, though it's much easier and faster to simply close and then reopen the software to reload their save from before the battle. Once they're able to get its HP into the red, they should start throwing Ultra Balls and continue to do so until the Pokemon is eventually caught. Putting it to sleep with a move like Yawn or Hypnosis first can make things a little easier, but this shouldn't really be needed to catch Heatran.