Once you've found the Magma Stone in Route 18 , you'll be able to use it to encounter Heatran, a Legendary found in Reversal Mountain. The easiest way to get there is to fly to Lentimas Town and head east into the mountain from there.
Once you're inside the mountain, head north through to the back of the first area. Move through the door leading east. Head south until you reach the south edge of the room, then head east. Continue east until you reach the Doctor standing in front of a doorway down some steps.
Move down the steps and enter the door. In the next room, head north down the steps. Attempt to move down the steps to the small little section that's lower than the other areas. You'll be stopped and asked to answer a question. Answer with 'Yes' and Heatran will appear.
Move up to it and interact with it to begin the battle. Don't forget to Save your game first, that way if you fail in catching (or defeating it, if you don't want to catch it), you'll be able to try again! You only get one shot otherwise!
Legendary Pokemon Battle
This guy is a massive pain to catch (unless you use the Master Ball of course). It has solid defences and high Special Attack, plus a powerful Fire-type move in Lava Plume . Water-types are best here, as they'll take reduced damage from Heatran's Steel- and Fire-type moves.
Once you've caught it or otherwise, that about wraps up our tour of east Unova, at least if you've been following our walkthrough precisely. From here, use an Escape Rope or backtrack to the entrance to leave the mountain, then head off to wherever interests you next.