Who do you get a arceus

Well usually you would apply for a special event, I think its called the zero flute, at spear piller you play the zero flute and a long stairway will appear you go up the stairway and you will see a platform with Arceus on it. Instead of using a master ball on Dialga or Palkia you should use it on Arceus and BAM, a nearlly invicable Pokemon at your desposal. In Pokemon Heart Gold and Soul Silver, if you take it to the Ruins of Alph, Arceus will transport you to the Sinjo Ruins Where you will meet Cynthia and she will take you inside the ruins for arceus to give birth to 1 of three Pokemon Dialga, Palkia, or Giratina each holding their special orb.