The reason why Mystra, the Goddess of Magic, invested a portion of her divine might into mortals is not known. One of the popular theories, and one that is gaining more support in light of the other goddess' during that period, is that Mystra foresaw the Time of Troubles (and her own passing at the hands of Helm) and chose to give some of her powers to mortals in order to ensure that her successor (the female mage, Midnight, as it turned out) would have a number of nearly immortal allies in the struggle against the schemes of the gods (the now dead Bane, Myrkul and Bhaal) who precipitated the Time of Troubles by stealing the Tablets of Fate.
The theory goes on to suggest that Mystra informed Azuth at approximately the Year of the Rising Flame (0 DR), more than 1,300 years ago before the Times of Troubles, that some of her power must be put into the hands of mortals who would then become known as Mystra's Chosen. The power would sleep within the bodies of those mortals, allowing Mystra only to call on it only with their permission. It would give the Chosen to innate ability to heal quickly, and would give them life spans far greater than those of ordinary mortals. Mystra speculated that these mortals might be able to call on her power and thereby gain some special abilities, but that these powers would not rival those of a deity. (See "Powers" below.)
The Goddess of All Magic then began to select mortals she thought to be suitable. One of the first was the young mage Elminister, and she also singled out a promising wizard named Khelben Arunsen. Both of them proved to be worthy and capable receptacles of her power, but Mystra's other early attempts in invest her power in living humans were unsuccessful, and she came to realize that only a very few mortals were of stern enough substance to contain such power within themselves without being destroyed or corrupted. Even though some people aside from Elminister and Khelben may have possessed the requisite strength, it is possible that having lived for years prior to being visited by Mystra had set them on a path from which they were not able to deviate. Whatever the reason, the problem needed to be solved. To get around the difficulty, Mystra devised a plan to use herself as a vessel to breed individuals who could be nurtured and acclimated to her power from the very beginnings of their lives.
For the father of these individuals, she picked the best example of human stock she could find: Dornar Silverhand, a nobleman and a former Harper who lived near Neverwinter. Mystra then possessed the body of Elue Shandur, a helf-elven sorceress whom Dornal was already attracted to. Mystra revealed her presence and her plan to Eleu, who happily and eagerly agreed to have the goddess share her body. Eleu had been reluctant, but under the influence of Mystra the woman became a seductress, and Dornal found his advances being suddenly returned with great favour.
Dornal and Mystra/Eleu were wed in the Year of the Drifting Stars (760 DR). The first of the seven daughters, Anastra Sylune, was born the following winter. Sylune's six sisters emerged at one-year interval thereafter: Endue Alustriel, Ambara Dove, Ethena Astorma (she prefers the name "Storm" these days), Anamanue Laeral, Alassa Shentrantra (known today as the Simbul) and Er'sseae Qilue. These siblings have become known in Realmsian lore as the Seven Sisters.
Dornal, who had been kept in the dark about his wife's true nature through the years (presumably because Mystra didn't want to risk losing his services), was disappointed and distraught by the time his sixth child was born; he had always wanted sons as well as daughters. More importantly, he was seeing his wife deteriorate right before his eyes. The strain of coexisting with the goddess all these years had turned Eleu into a withered shell - in essence a lich, clinging to life only because Mystra's power was within her.
When Eleu was carrying the seventh child, Dornal consulted a priest who told him his wife had been possessed by a entity of great magical power. To spare both of them any further agony, he attempted to slay his wife's physical form by severing her head from her body.
As soon he had done this, Mystra was forced to reveal herself to him, and she went on to explain her scheme. Just as she had worried would happen, Dornal was aghast at how he and wife had been used by the goddess. He turned his back on the corpse of his wife, abandoned his lands and his children, and vanished into the North. Mystra bore him no ill will, and in fact protected him for the final 30 years of his life. When Dornal finally did meet his end he called out to Mystra, and the Goddess granted his continued existence as her servant. Now known as the Watcher, Dornal Silverhand travels the world unseen by mortals continuing mission to locate candidates to swell the ranks of the Chosen and to identify possible threats to Mystra and her minions.