Chromium Rhuell
Chromium Rhuell was one of the powerful Elder Dragons. His death drove his friend Ravidel insane, thereby indirectly causing the Planeswalkers' War.
Life
Ages ago the Elder Dragons ruled supreme; but with so many beings of so great power living together, war was inevitable. In the Elder Dragon Wars, they nearly wiped themselves out, leaving only a few real Elder Dragons alive. The losers of the war were either killed or stripped of their powers, wings, and limbs and turned into the Elder Land Wurms. The winners set out across the multiverse and became the ancestors of all modern dragons, drakes, and viashino. The only known survivors of that war were Chromium, his siblings Nicol Bolas, Arcades Sabboth, Palladia-Mors, and their cousin Vaevictis Asmadi.
Chromium and his life partner Piru eventually became servants of the planeswalker Geyadrone Dihada. One day they were summoned by her to fight Dakkon Blackblade. Dakkon tried to take control over Chromium and to force him to kill Piru, but he fought back and drained most of Dakkon's life away. With the last of his power, the planeswalker teleported Chromium to an unknown location. Piru was less fortunate however, for shortly afterwards Dakkon was reunited with the Blackblade and used it to kill the dragon.
It is unknown how Chromium ceased his service to Dihada, or how he dealt with Piru's death. The next time he was seen he posed as "Ham," the Tickery Man, and was a member of the High Council of Mors Ridge who was in charge of casting the spell Trickery, making sure his sister Palladia-Mors remained sleeping beneath Mors Ridge to keep her from attacking humans.
Shortly after this, Chromium once again pledged allegiance to a planeswalker, this time to Faralyn. In his service, Chromium befriended the planeswalker's spellsquire, Ravidel. During this time, Arcades Sabboth was forcibly summoned by Leshrac to fight Kristina, and in effect was killed by her.
As the Shard of the Twelve Worlds closed, Chromium was caught inside it. In 2934 AR, Faralyn called for a meeting with several other planeswalkers that were caught inside as well, the so-called Summit of the Null Moon. Tensions were high however, for the people present all had a history with one another: Leshrac and Kristina were among those who where invited. To make matters worse, Faralyn had secretly betrayed them. He, Leshrac, and Tevesh Szat planned to start fighting between the people present, hoping that the death of a planeswalker would provide enough energy for them to escape the Shard. No planeswalker died, but Szat killed Chromium, who apparently was powerful enough to allow them to escape. During the battle, Ravidel was killed as well, but Kristina and her lover Taysir joined forces to return him to life. Instead of being grateful, Ravidel was furious that they saved him instead of Chromium. Ravidel plotted vengeance for ages, gradually becoming more and more insane. Eventually this would lead him into starting the Planeswalkers' War.
Ravidel took Chromium's corpse and fashioned it into what he called a Planar Barge, which he used to carry his army in while planeswalking.
Chromium and Chromium Rhuell?
The dragon in the art of the card Chromium does not look much like the Chromium Rhuell from the comics. Both are dragons and both have chrome-like scales, but the version from the comics is far thinner and more robot-like. Also, in the comics he is always called either "Rhuell", or "Chromium dragon", suggesting he is called Rhuell, while Chromium is a type of dragon. This has lead some people to think they might actually be two different dragons. Some people have speculated that the original Elder Dragon Chromium spawned a race of Chromium Dragons and that Rhuell was a Lesser Elder.
However, the article from Elder Dragons comic reveals that they are one and the same character. Changes in appearence can be explained by shapeshifting abilities.
The difference in appearance can also be explained by the different artists' styles.