Pokémon Diamond and Pearl/Mt. Coronet south

Mt. Coronet (Japanese: テンガンざん Mt. Tengan) is part of a long range of mountains that divide the Sinnoh region in half. Most of it is a deep cave, though there are several above-ground areas. The name is a reference to the word corona. This time, Team Galactic has headed up here to accomplish their goal of creating a new world. Of course, you have to stop them.

The mountain's peak is the Spear Pillar, where Dialga or Palkia are awakened, and where the Hall of Origin is reached.

Climbing the Mountain
Take the entrance from Oreburgh City or Hearthome City. If you take the wrong entrance, a lost Galactic grunt will tell you where to go. Surf on the pool, then Rock Climb to the stairs. In the next room, the wild Pokémon get much tougher. Evolved forms of Pokémon will start showing up. Head for the back of the room, then keep climbing. From now on, Galactic Grunts will start appearing in the caverns, so deal with them. Eventually, you will find an exit, leading toward…

The Summit
No, not yet. Out here, it snows just as hard as Route 217. Wild Abomasnow can be caught, if you wish. To progress further, go through the grass into the cave entrance. Rock Climbing on the right will be a dead-end for now. Go through the short cave to find yourself in another snowy area. It doesn't snow heavily here, but there is another set of paths you can take. The cave easily reached leads to a big room full of Galactic grunts, but one is blocking the way. To reach the Spear Pillar, take the entrance to the left (it might be hard to find). The rest of the path is straightforward, but be wary of the wild Pokémon levels increasing.

The Spear Pillar
Battle the two grunts, then save (since several events are going to happen). Cyrus, using the Red Chain, summons Dialga (Diamond) or Palkia (Pearl). Go forward and Mars and Jupiter challenge you (to slow down your efforts). Luckily, your rival shows up to help.

After you beat them, Dialga/Palkia starts the job of destroying the world. However, Uxie, Mesprit, and Azelf come and stop it, breaking the Red Chain in the process. Your rival heals you, and Cyrus tries to take his vengeance on you.

After Team Galactic leaves, you friend and Professor Rowan come and tell you to help the Pokémon standing there. Save and get in a battle with Dialga or Palkia.

After you catch/defeat Dialga/Palkia, you will be able to leave. However, go past where it stood to get an item that increases the power of your legendary's moves (that are the same type as Dialga/Palkia).