Chromatus

Chromatus is the first and only successfully formed chromatic dragon. Chromatus' five heads grant him the power of all five dragonflights. Though the five-headed dragon appeared dead because Nefarian never managed to give it life, the Twilight Father managed to breathe life into the creature through the use of a surge needle. It was later rendered inanimate by the united powers of the Dragon Aspects, and its body was stored in an arcane prison.

Background
The monstrosity was created by Deathwing's son, Nefarian. Nefarian had tried to create a new dragonflight that would combine the powers of all five of the other flights. The experiments were considered failures as many whelps had died before hatching or were deformed in many ways. Chromatus was Nefarian's final experiment. Chromatus exemplified the pinnacle of all Nefarian had learned through his various experiments, but Nefarian was slain before he could give Chromatus the spark of life. Years later the Twilights Cultists brought the body of Chromatus all the way to Northrend, knowing that they needed vast stores of arcane energy to give him the spark of life.

To revive Chromatus the Twilight Father made a pact with the blue dragon Arygos. After being betrayed and killed, Arygos' blood was used to siphon the magic from Azeroth into the Nexus and from the Nexus into Chromatus. Slowly, the monstrous creature began to move, each head slowly coming alive. Shortly after being animated Chromatus joined the battle against Kalecgos and the blue dragonflight. With the immense power of Chromatus the blue dragons had to retreat. At Wyrmrest Temple, Chromatus became interested in the captive Kirygosa and wished to mate with her so she could bear his eggs to make the first generation of stable chromatic whelps. However Kirygosa managed to escape just in time. As Kirygosa explained the situation with Chromatus to Ysera, Alexstrasza and Kalecgos, Ysera realized that her vision of the Hour of Twilight might be a result of Chromatus slaughtering the Aspects. So all three Aspects decided that Chromatus had to die before he would grow too powerful and attacked him.

After learning about Ysera's vision, the Aspects decided to attack a different colored head than their Dragonflight. Ysera focused on the bronze head. Now fully Awakened, Ysera appeared to be a force to be reckoned with, her flight pattern incredibly unpredictable, and staying two steps ahead of the bronze head of Chromatus. Alexstrasza fought the most intelligent head, the blue one, and Kalecgos began fighting the red head. Suddenly, the black and green heads focused on attacking Ysera along with the bronze one. One of Ysera's forelegs was caught in a Shadowflame Attack, while the bronze head spat sand that scorched her skin, and the black head bit her wing, taking a chunk of it. Ysera pulls back to heal herself, and then all of the heads begin focusing on the green Dragon Aspect. Finally, the bronze dragons, with Nozdormu at their front, joined the battle. However, the first thing the Timeless One did was to urge them all to retreat, stating that if they had persisted, they all would have died. While Thrall is not a Dragon Aspect, he works as a shaman with the Earth, and offered to give his spirit of Earth in the battle with Chromatus, so that all five Spirits of the Aspects were whole once more. As he prepared a ritual to bind their spirits, the Twilight Dragonflight attacks. The four Aspects renewed their attack upon Chromatus, but this time with a much more unified power. Chromatus faltered for a moment, all of its heads reared back to attack, exposing its chest. It was in this moment that all four Aspects unleashed the a pure white hue which brought Chromatus to fall.

Concerned that some other dark power might try to revive Chromatus after its defeat, the dragons attempted to destroy the corpse. They failed. Some powerful spell, probably woven into the dark marriage of magic and technology that had animated him in the first place protected the body from all their efforts to obliterate it. Kalecgos stated that Malygos created a number of Arcane Prisons during the Nexus War, and Alexstrasza decided that representatives from all the flights would watch over the body of Chromatus, as he is not dead.

Description
Chromatus' body is enormous, far larger than even a Dragon Aspect's. It looks misshapen with its color of an ugly purple bruise on pale skin. In some places, bones are visible. Scales had fallen off, showing skin that was healthy and skin that was decayed. Each of the heads was a different color and seems to have something amiss with it: a missing ear or an oozing eye. In Thrall's eyes, Chromatus was not just a dragon, but such a dragon as might have been conjured by a Forsaken's worst nightmare.

Each head of Chromatus has a brain that knows all the skills of its flight, and is able to attack as separate entities or act in eerie unison, with such furor that the entire blue dragonflight had become rattled and panicky. Sometimes his wings would not bear him, yet the blue flight was unable to stand against him and the twilight dragonflight. His green head fixed Ysera with an intense stare, attempting to send the green Dragon Aspect one of her own nightmares. The bronze head can breath out sand. Instead of five separate hues from five separate heads, the flames the creature can spew are dark violet and far more powerful.

Quotes

 * Chromatus (to Kirygosa): “Fear not, little blue. Your brother lives—within me. We feel our kinship."


 * Twilight Father: "Now ...now you see. Glorious, isn’t it, this coming doom in all its inevitability? It doesn’t matter if the blues now have an  Aspect.  It  doesn't matter if Ysera is awakened, or if Nozdormu is found, or even if the Life-Binder herself returns. Chromatus lives ... so that the Aspects will die."


 * Chromatus' red head (to the Twilight Father): "I am not your friend, Twilight Father. Nor am I your child or your servant. We both serve the mighty Deathwing, whom my father made me to serve, and that is our only commonality."


 * Chromatus' bronze head: "Time passes. I am the only success of my father's experimentation. Perhaps a more stable — a more... traditional method of creating chromatic whelps will ensure that they are strong enough to survive. I the father, and the mother of the last child of Malygos? Yes... our children will be stronger.."


 * Chromatus (last words): "You cannot stop me! I know all your secrets! It took all of you to even attempt to destroy me! Think you Deathwing will be easier? He will rip this world apart to crush you for what you do! And I will be there with..."

Trivia

 * "Chroma" is derived from the Ancient Greek "χρώμα," meaning "color."
 * The character may have been inspired by Tiamat (from Dungeons & Dragons) or Takhisis (from Dragonlance) - dragon gods with five heads representing the chromatic dragonflights of those settings (red, blue, green, black, and white).

Speculation
It is possible Ultraxion was originally meant to be Chromatus, possibly because animating a chimaera-like being turned out too hard for the developers. Chromatus just disappeared from the story, and Ultraxion drops a chromatic drake mount. It could also explain why the Twilight's Hammer was present in the Eye of Eternity, as Chromatus was originally imprisoned in an Arcane Prison, technology of the Blue dragonflight.