The Dragons Primeval is a collective term for the five greatest dragon lords: Darigaaz, Rith, Treva, Dromar and Crosis. Thousands of years before the Phyrexian Invasion and the Rathi Overlay, these five dragons ruled, undisputed, over all other dragons, who in turn ruled all the other mortal races. Another name for this collective is the Ur-Dragon
Though each were extremely powerful on their own, together they were nearly invincible. However, one of the human kings managed to trick Darigaaz by luring him into a shivian volcano and having his greatest mage, Averru, cause the volcano to erupt. While encased in magma, Darigaaz was cut off from the other primevals, weakening them and allowing the mortal races of Dominaria to revolt and seal the other four: Rith inside the Heart of Yavimaya, Treva in the Library of New Argive, Dromar in a cavern deep in the Vodalian Sea and Crosis in the tar pits of Urborg.
They remained sealed until just after the Rathi Overlay, when the Phyrexian army raided Darigaaz's tomb and attempted to kill him. But even though they destroyed his body, Darigaaz was not killed. Having the power to be reincarnated, Darigaaz had been reborn, over a hundred years before, as Rhammidarigaaz of the Shivan Dragons.
Having no memories of his previous life and believing the red Primeval to be dead, Rhammidarigaaz was convinced by planeswalker, Tevash Szat, to raise the other four. Believing that the dragon lords will help the other mortal races will help fight the phyrexians, Rhammidarigaaz commands the dragon races to travel to Yavimaya Forest.
In Yavimaya, Rith manages to call to Rhammidarigaaz's mind from inside her prison. Believing that Rhammidarigaaz is taking the dragon races on a wild goose chase, one of the other dragon commanders challenges him to single combat. The mutinous dragon, Rokun, is killed and his blood spills on the heart of Yavimaya and Rith's prison collapses and the sealed dragon lord was released.
In the Card Game
In the Magic: the Gathering card game, the Dragons Primeval were all printed in the Invasion set as a cycle of multi-colored 6/6 legendary dragons.
These cards were each accompanied by an attendant, a 3/3 golem that could sacrifice for three colors of mana; a charm, a spell that you could choose one of three effects; and a lair, a land that could produce three different colors of mana.
The Chaos Primevals
In the Planar Chaos set, five more multi-colored dragons were released. Unlike the other legendary creatures from the set, these were not the same characters in different colors. These dragons were a whole new set of Primevals, possibly from an alternate history were the originals were killed off or overthrown.