Worldbinder Runelords book 6

Fallion and Jaz, sons of the Earth King Garborn, have returned from their long exile. Yet they have arrived under cover of darkess, fugitives in their own kingdom. There seems little left to save, but Fallion's heritage has given him a powerful calling to heal not just this land, but all lands - if he can master the flameweaver's talent that threatens to consume him. But he has an ancient enemy, skilled in the ways of evil, and she initiated the catastrophe that doomed the worlds of men.
Many ages ago, a perfect world was split into fragments, and each fragment was but a distorted image of this golden age. If Fallion could but unite all these splinters, he could recreate the One True World of Legend, and heal the sickness in the realm. But his adversary wants the One World for her own ends, and an army of darkness is on her side.