Many millennia ago, a meteor from space which threatened to destroy the world was stopped by Arceus who intervened to protect the world. While attempting to protect the world, Arceus was badly injured and Damos (&#12480;&#12514;&#12473;) took care of it. As gratidude, Arceus promised to help make the desolate land of Mich&#299;na rich and prosperous so it gave Damos the Jewel of Life (&#21629;&#12398;&#23453;&#29577;). 
Arceus let Damos borrow 16 parts of its body to create the Jewel of Life which were to be returned to it on a specific date. Arceus was betrayed when it was not returned and it vowed to judge humans when it awakens. 
Many millennia after these events, Satoshi, Hikari and Takeshi arrive in the riverside town of Mich&#299;na (&#12511;&#12481;&#12540;&#12490;) which is surrounded by nature's beauty. They decide to go the ruins of Mich&#299;na which are at the summit of a large rock. There is a large and beautiful lake at its shores and Satoshi and friends are fascinated by the lake, but suddenly, waves begin to form in the lake and tornado of water begins to wreak havoc. 
A descendant of Damos, a girl called Sheena, runs out from the ruins of Mich&#299;na. Sheena saw what was happening and she summoned Dialga and she says "Dialga, please!" and Dialga manages to stop the tornado. 
Satoshi and his friends finally get to see the ruins and Kevin and Sheena tell the old legend of Mich&#299;na to Satoshi and his friends. Moments after they explain the legend, the fighting between Dialga, Palkia and Giratina becomes fierce and the ruins of Mich&#299;na are destroyed. Arceus is awoken... 
(Source: Pocketmonsters)