You need the Pokemon Arceus, It has to be caught in Diamond, Pearl, or Platinum then traded over to HeartGold. After you get it in HeartGold you put it the first in your party and leave all your other Pokemon in the PC then go to the house in the Ruins of Alph it will start the event that will let you get one of the three cover Pokemon of the D/P/Pt games.
(I suggest getting Giratina because the orb it holds cant be traded from Platinum so it's the only way to get the Origin Forme )
Shinto Ruins is only obtainable if you have an Arceus.
Shinto Ruins
take arceus to the unown ruins
event place.
He goes to the Shinto Ruins in Pokemon HeartGold and SoulSilver. You will see him if you do the Arceus subquest
no
unkowns {pokemon}
The Ruins of Alph.
First, you need an Event Arceus and you go to Shinto Ruins. Cynthia will be there as well. Then Arceus can create either Giratina, Dialga or Palkia. They will be at Level 1.
in Pokemon heartgold ruins of alph in Pokemon soulsilver ruins of alph
start smashing rocks where the ruins r
In the English version, it's Sinjoh ruins. You have to have an arceus from the event and trade it over from DP.