From Bulbapedia, the community-driven Pokémon encyclopedia.
The Red Chain (Japanese: あかいくさり Red Chain) is an unobtainable key item in Pokémon Diamond, Pearl, and Platinum, created from crystals drawn from the Sinnoh's lake guardians. It is used by Cyrus to take control over the legendary Pokémon Dialga and Palkia so as not to reduce their power by capturing them in a Poké Ball, with the ultimate goal being to destroy the universe and make a new one without spirit.
In the games
The Red Chain is mentioned throughout the Team Galactic Headquarters in Veilstone City, during the player's infiltration late in the game. Mars, Jupiter, and Saturn succeed in capturing Mesprit, Uxie, and Azelf, respectively, bringing them to the Headquarters, where Charon extracts the gems from their bodies to create the Red Chain.
Cyrus duplicates the chain through technological means (it is not explained how, just that it was done), and, despite being defeated by the player, travels with Mars, Jupiter, and a plethora of Grunts to the Spear Pillar at the top of Mt. Coronet. In Platinum, blasting away a priceless, ancient painting that depicts the creation of Sinnoh to get there was required.
Mars and Jupiter manage to hold both the player and his or her rival back just long enough for Cyrus to summon both Dialga and Palkia, bringing them under his control. Though the lake guardians attempt to interfere, the power of the two legendary Dragons is far too much, and nothing can be done. Giratina, enraged, and feeling the oncoming destruction of the Pokémon world from its own world, creates a portal between the two in front of Cyrus, breaking his hold over Dialga and Palkia, and draws him into the Distortion World to prevent further destruction of the dual universes. All is still not well, however, and although Cyrus has been stopped and the Red Chains destroyed, the very fabric of spacetime is in danger due to Giratina's portal between the two worlds, leaving it up to the player to find and defeat Cyrus one last time and close the portal between the worlds to prevent their mutual annihilation.
While the Chain cannot be obtained through normal gameplay, beta data exists which can be accessed through hacking. An in-bag description can be found, and reads as follows:
- A mythical chain that is said to link the legendary Pokémon that created the Sinnoh region.
It is said that the Red Chain summons Dialga and Palkia when it is put together.
From Cyrus's PC entries
In Platinum, Cyrus's computer details information on the Red Chain:
- The Pokémon of the lakes and Mt. Coronet are somehow connected. Capturing the Pokémon of the lakes will free the Pokémon of Mt. Coronet. From the Pokémon of the lakes, crystals can be extracted to create a Red Chain.
Using that Red Chain, the mythical Pokémon can be summoned and shackled to do as we command...
In the anime
The Red Chain wrapping itself around Palkia
Like in the games, the ultimate goal of Cyrus and Team Galactic is to capture Dialga and Palkia with the Red Chain. In Unlocking the Red Chain of Events!, The Needs of Three!, and Battle Finale of Legend!, Cyrus finally succeeds in doing so, and as it wraps itself around their bodies, the chain transforms itself into a red-colored version of the ring that surrounds Arceus's own body.
Unlike the games, Uxie, Mesprit, and Azelf were not needed to create the Red Chain. Instead, it was created from the meteorites Team Galactic stole from Veilstone City.