Red Chain
The Red Chain is a special key item, in Pokémon Diamond, Pearl, and Platinum, created from the gems of Sinnoh's legendary trio: Uxie, Mesprit and Azelf. It is used by Cyrus to bring out either Dialga or Palkia, in order to complete his dream of creating a new world.
Description
A mythical chain that is said to link the legendary Pokémon that created the Sinnoh region. a chain of red gems from the 3 lake pokemon used to gain control of dialga or palkia. cyrus uses it to make a new world.
Location
The item is, in fact, unobtainable in Pokémon Diamond and Pearl. This is because Cyrus uses the item himself in the story, and by the time Dialga/Palkia is summoned, the item has lost its purpose. The key item was created in the Pokémon Diamond and Pearl prototype and still remains in the games coding. It can still be obtained by hacking into the game's coding with a hacking device, but it serves with no real function. It can be assumed that the Dialga/Palkia event was going to be started by the player instead of Cyrus, or that it was created in order to beta test the game before Team Galactic was put into the game.
After the three Pokémon of the lake appeared at the Spear Pillar to Dialga/Palkia, the Red Chain crumbles away and the legendary dragon is no longer under Cyrus' control.