Lord Ao created the universe that now holds the world of Toril. After this creation, there was a period of timeless nothingness, a misty realm of shadows that existed before light and darkness were separated. Eventually this shadowy essence coalesced to form twin beautiful goddesses, polar opposites of each other, one dark and one light. The twin goddesses created the bodies of the heavens, creating Chauntea, the embodiment of the world Toril. Toril was lit by the cool radiance of the goddess Selune, and darkened by the welcoming embrace of the goddess Shar, but no heat yet existed in this place.
Chauntea begged for warmth that she might nurture life and living creatures upon her form, and this caused the twin goddesses to become divided in intent. The two fought, and from their divine conflict the deities of war, disease, murder, death, and others were created.
Selûne reached beyond the universe to a plane of fire, using pure flame to ignite one of the heavenly bodies to that Chauntea would be warmed. Shar became enraged and began to snuff out all light and warmth in the universe. Desperate and greatly weakened, Selûne tore the divine essence of magic from her body and hurled it at her sister, tearing through Shar's form and pulling with it like energy from the dark twin. This energy formed Mystryl, the goddess of magic. Composed of light and dark magic. Composed of light and dark magic but favoring her first mother, Mystryl balanced the battle enough to establish an uneasy truce between the two sisters.
Shar, who remained powerful, nursed a bitter loneliness in the darkness and plotted her revenge. Selûne waxed and waned with the light, but drew strength from her allied daughters and sons, and even interloper deities from other planes. Their battle continues to this day.