On Tarkir, 1,280 years ago, Sarkhan Vol rescued Ugin from death after his climactic battle against Nicol Bolas. Ugin remained alive, albeit in a dormant, cocooned state. As a direct result of this action, the dragon tempests on Tarkir persisted rather than petering out. The dragons thrived rather than going extinct. In fact, they emerged victorious in their war of supremacy against the nascent clans and their khans.
The khan-led clans fought the dragons as best they could. But one by one, each of the ancient humanoid khans fell, and their clans fell with them—an event more than a thousand years ago that is now known as the Khanfall. In place of the old humanoid clans rose the five dragon clans, named after their dragonlords: the elder dragons Dromoka, Ojutai, Silumgar, Kolaghan, and Atarka.
The dragon clans did not wipe out all of the humanoid races. After the khans were defeated, the dragons turned their attentions to each other. The dragons allowed humanoids to follow and support them in their struggle against one another, forming five new dragon-led clans. Almost all humanoids now live as part of one of these five clans, serving a variety of purposes for their dragon overlords. Life can be hard in the dragon clans; the highest-ranked humanoid still ranks lower than the lowest-status dragon. Still, membership is still the safest life for humanoids. This is the world that Sarkhan's actions created: a world where dragons truly rule. Even Zurgo - the Mardu khan in Sarkhan's original timeline - must bow to a dragonlord.
The defeated clans are ruled by five legendary dragonlords. Over time, the clans have come to embody the predominant aspects of their draconic masters and they represent these strengths with draconic symbols. Each clan bears the name of its dragonlord and represents the corresponding draconic aspect: Endurance, Cunning, Ruthlessness, Speed, and Savagery.