The Thran were a utopian culture of humans whose technology level was much more advanced than any other Dominarians. Their artifact devices ran on powerstones which had stable artificial planes in them. They were introduced in the Urza's Saga set, with such cards as Thran Quarry and Thran Dynamo amongst others.
The Thran Empire, centered in their capital city of Halcyon, encompassed much of the main planet of the plane of Dominaria. Their culture was idyllic and pacifistic, and the Thran people lived very lush and comfortable lives. This all changed when a group of rebellious Untouchables, people who had been exiled from Halcyon to live and die in the Caves of the Damned, for one reason or another, attacked the city's mana rig, or power source, effectively, situated in a volcanic crevice above the Caves of the Damned and below the City of Halcyon. The Genius of Halcyon, a man named Glacian, was hurt in the raid on the mana rig, and contracted a disease which none of the city's healers were able to cure; their arcane healing magic only worsened the disease.
The Thran began to suffer from this disease known as Phthisis, which an ex-Thran healer named Yawgmoth determined was caused by the powerstones which the city ran on. The disease changed the way the Thran Empire worked; it spread like the Plague and Yawgmoth had the only known cure—a serum consisting of metal components in solution which blocked the flow of mana (natural energy) through the body, a process called phyrexis. Yawgmoth became the hero of Halcyon for discovering a way to fight the new disease and gained political power from it. Yawgmoth, driven primarily by a desire for power, used his campaign against the Phthisis to manipulate things in his favor and eventually gain control of the Thran capital city of Halcyon. During his quest for power, he took control of a plane he named Phyrexia for his term phyrexis which meant regeneration.
The Thran-Phyrexian War began when Yawgmoth returned to conquer the Thran and the plane of Dominaria. Yawgmoth, in control of the city of Halcyon and its elite guard, along with his new army of Phyrexians, waged war against the Thran coalitions that opposed him. During this battle, Yawgmoth took control of the Null Moon, a giant, movable structure which could control all the artifacts in the Thran Empire, and used it to cripple the Thran Coalition's effort. Yawgmoth used the Null Moon to drop powerstone chargers, great dooms-day weapons which sucked energy off the land and everything on it or around it to charge a single powerstone. Yawgmoth was finally defeated when he and his followers were sealed in Phyrexia by a portal-device powered by a powerstone. For the Thran, the victory was pyrrhic. Their civilization had been damaged too greatly to recover, and soon collapsed.
Thousands of years later, all that was left of the Thran was their advanced artifacts, which were scavenged by later civilizations for study. Two such salvagers, the brothers Urza and Mishra, researched these remnants and during a visit to the Cave of Koilos, the powerstone powering the device sealing Phyrexia was broken. Each brother received one half of the powerstone (named the Mightstone and Weakstone), and their greed for both halves of the stone and their animosity for each other eventually led to The Brothers' War. Much later, the opening of the gate to Phyrexia would lead another Phyrexian invasion of Dominaria.
The Thran, written in 1999 by J. Robert King, further discusses this race.