How to Get Inside the Snowpoint Temple in Pokémon Diamond and Pearl

Gaining Access to the Snowpoint Temple

Beat all 8 Gym leaders. Roark, Gardenia, Maylene, Crasher Wake, Fantina, Byron, Candice, and Volkner must all be defeated for you to obtain the National Pokedex, which is required to enter the temple.

Beat Elite Four at Pokémon League. You must beat Bug, Ground, Fire, and Psychic to be able to upgrade to the National Pokedex.

Beat the Pokémon League Champion, Cynthia. Once you defeat Cynthia, you will wake up in your house. Your mom will congratulate you on your win.

Check your Sinnoh Dex. After fighting all the trainers, you should have seen all 150 Pokemon, filling your Dex. If you are missing any, go back and fight any trainers you may have skipped. You don't need to have captured all 150 Pokemon. Seeing them is enough. If you are playing platinum, then you must see all 210 Pokemon.

Go to Professor Rowan's lab once you have filled your Dex. He will give you the National Pokedex. You will now be allowed access to the Snowpoint Temple, where you can fight legendary Pokémon Regigigas.

Fly up to Snowpoint and try to enter the temple. The temple is in the northern-most point of the Sinnoh region. The trainer at the door will try and keep your from entering the temple, but the Snowpoint gym leader, Candice, will come outside and say you can go inside!

Awakening Regigigas

Make sure you have Regirock, Regice and Registeel in your Pokémon party. Trade with a friend or migrate them from Pokémon Ruby, Sapphire or Emerald version. You will not be able to fight Regigigas without them. You should also have: A strong Pokémon (Preferably Breloom or Parasect) 40+ Dusk balls

Navigate to level B5F, where you will find the statue of Regigigas. When you approach the statue, he will awaken and you can fight him. He is strong but has the Slow Start ability, so use your first five moves to lower his attack and speed stats. Make sure you save before fighting. If Regigigas blacks you out, you run out of balls, or you kill him, you can just switch your DS off and on and try again. If you kill him and you haven't saved your game, you must defeat the Champion again so he will respawn.