Arien was the Driver of the Sun, a Maia of the Ainur, therefore created by Eru and deathless.
She was from the beginning a spirit of fire, never deceived into the service of Melkor as had been the Balrogs. Her eyes were too bright for even the Eldar to bear. The maiden feared not the heats of the Tree, Laurelin, for she could not be hurt by them. Before Melkor and Ungoliant killed the Trees, Arien tended the golden flowers in Vana's gardens, watering them with the bright dews of Laurelin.
Arien was chosen by the Valar from the Maiar to guide the vessel of the Sun (Anar in the Quenya language). This vessel was the final fruit of the Tree, Laurelin, brought forth past hope from the healing tears of Nienna and the powerful song of Yavanna. At the same time, the Tree Telperion bore the last silver flower, which became Isil. Yavanna gave the fruit and flower to Aule, Manwe hallowed them, then Aule and his people made the vessels to hold and preserve the radiance. The light could not recall the light of the Trees before the poisonous touch of Ungoliant, that light continuing only in the Silmarils.
The Valar gave the Anar and Isil to Varda, who gave the vessels the power to cross the lower regions of Ilmen (the region just above the air where stars exist) upon appointed courses above the girdle of Arda (the Earth) from West to East, and then return. They were to be ever aloft, passing in opposite directions. Time was to be reckoned by the time of the mingling of lights above the middle of the flat earth.
For the Valar remembered the darkness of the lands of Arda, bereft of the Trees, the Noldor in exile, and the Atani (Mankind) who remained yet in an unknown place by the waters to be awakened, as Manwe knew, in a time soon to come. The Valar used light to hinder the deeds of Melkor, to avoid a war that could destroy men before they began existence, as it would be worse than the previous war upon Utumno, with the newest inhabitants of Arda even weaker than the elves had been, in withstanding fear and tumult.
So Arien forsook the form and raiment she had worn in Valinor and was as a terrible, splendid flame. The first time Arien guided Anar into the sky, Melkor took his servants and retreated from the light into the deeps of Angband, hiding his land from the Daystar with reeks and dark clouds. Thus was a quiet but glorious victory achieved.
But the course of Anar was soon afterwards changed, thanks to Tilion, Este, and Lorien (Irmo). For the rest of the story, see "Tilion".