"Too bright were the eyes of Arien for even the Eldar to look on, and leaving Valinor she forsook the form and raiment which like the Valar she had worn there, and she was as a naked flame, terrible in the fullness of her splendor."
― Quenta Silmarillion, "Of the Sun and Moon and the Hiding of Valinor"
Arien was originally a spirit of fire whom Melkor failed to corrupt.[1]
In Myths Transformed, which changed the timing of the Sun to have been primordial to Arda's inception, Melkor wanted to claim Arien as a wife,[7]:380-1 but when she refused him, he attempted to ravish her.[7]:405 However, she released her spirit and burned Melkor leaving him ashen and grey. The Sun was then left without a guide and caused uneven seasons and periods of excessive heat or cold in Arda.[7]:381 In other abandoned writings, she was a Maia of Varda.[7]:380
Arien was chosen by the Valar to carry the vessel of the Sun because she was the only one who was not afraid and could bear the heats of the great Tree Laurelin and so was unhurt by it.[1]
The History of Middle-earth, Vol.5: The Lost Road and Other Writings, VI. Quenta Silmarillion, Chap.6: "Of the Sun and Moon and the Hiding of Valinor"