History of the Sisters of Light and Darkness
This was the birth of the world and the heavens. After Lord Ao created Realmspace, there was a period of timeless nothingness, a misty realm of shadows before light and dark were separate entities. Within this dim chaos stalked 13 lords of shadow, the Shadevari - whether they came form elsewhere or are children of the shadow itself, none can say. Eventually this primordial essence coalesced into twin beautiful goddesses who were yin and yang to each other; they were so close they thought of themselves as one being. The Two-Faced Goddess created the heavenly bodies of the crystal sphere and together infused them with life to form the Earthmother, Chauntea. (Although Chauntea has since contracted her essence to encompass only Abeir-Toril, in the beginning she embodied all matter in Realmspace).
This new universe was lit by the face of the silver-haired goddess, who called herself Selune, and darkened by the welcoming tresses of the raven-haired goddess, Shar, but no heat or fire existed within it. Chauntea begged for warmth so that she could nurture life and living creatures upon the planets that were her body and limbs, and the two sisters-Who-Were-One become divided, as for the first time they were of two minds. Silvery Selune contested with her dark sister over whether or not to bring further life to the worlds. During this great conflagration, the gods of war, disease, murder, and death, among others, were created from residues of the deific battle.
At one point during the battle, Selune seized the advantage and reached across time and space to a land of eternal fire. Fighting the pain of the blaze, which burned her sorely, she broke off a fragment of that ever-living flame and ignited one of the heavenly bodies so that it burned in the sky and warmed Chauntea. Incensed, Shar redoubled her attack on her injured twin and began to snuff out all light and heat throughout the crystal sphere. Again Selune gave of herself and tore the divine essence of magic from her body, flinging it desperately at her sister in defense of life in the sphere.
This essence entered Shar, ripped an equal portion of energy from her, and reformed behind her as the goddess of magic, known now as Mystra, but then as Mystryl. Though Mystryl was composed of both light and dark magic, she favored her first mother Selune initially, allowing the silver goddess to win an uneasy truce with her more powerful, dark twin. Consumed by bitterness at her defeat, Shar vowed eternal revenge. The twin goddesses contested for eons as life struggled into existence on Toril and the other planets under Chauntea's watchful gaze. Shar remained powerful, but bitterly alone, while Selune waxed and waned in power, often drawing strength from her allied Daughters and sons and like-minded immigrant deities.
Over time, Shar grew strong again, aided by the shadevari who preferred night to blinding light and who stalked the Realms seeking to meld light and dark into shadowy chaos once again. Shar's plot to reform the world after her own desires was undone when Azuth, the High One, formerly the greatest of all mortal spellcasters and now consort to Mystra (incarnate successor to Mystryl), found a way to imprison the shadevari in a pocket-sized crystal sphere located beyond the edges of the world by creating the illusion of a realm of shadows.
The Lords of Shadow were drawn to investigate, and before they discovered the trick, Azuth imprisoned the shadevari with the Shadowstar, a key of shadows forged by Gond. The High Lord then hurled the key into the endless reaches of the cosmos allowing life to flourish on in Chauntea's loving hands.