Shar, the Mistress of the Night , was the goddess of darkness and the caverns of Faerûn, as well as a neutral evil greater deity. A counterpart to her twin Selûne, she presided over caverns, darkness, dungeons, forgetfulness, loss, night, secrets, and the Underdark. Among her array of twisted powers was the ability to see everything that lay or happened in the dark. Shar's symbol was a black disk with a deep purple border. Shar was also the creator of the Shadow Weave, which was a counterpart and attack upon the Weave, controlled by Mystryl and her successors before both of the Weaves fell into ruin during the Spellplague.

According to one of the most ancient myths of the creation of the world and the heavens, after the universe and its crystal sphere were created by Lord Ao, there was naught but the primordial essence, the protoplasmic raw stuff of existence. Described as chaos and timeless nothingness, the sphere was filled with no more than dim misty shadows, neither light nor dark, for such things had not yet separated. (All that moved here were the Shadevari, the thirteen lords of shadow, whose origin, whether from elsewhere or from the shadow itself, is unknown.)

In time, Shar coalesced from the primordial essence, alongside her twin sister, Selûne. The goddesses were beautiful, identical but polar opposites, raven-haired and silver-haired, one representing the dark, the other the light in the manner of yin and yang. Yet they were so close they saw themselves as one being, known later as the Two-Faced Goddess or the Sisters-Who-Were-One. They complimented each other and brought order out of the chaos.

Together, they created from the cosmic ether Abeir-Toril and the other heavenly bodies and infused these worlds with life. In the process, they formed the goddess Chauntea (at that time, the embodiment of all matter in Realmspace, later only of the world of Abeir-Toril), whom they worked with to bless the worlds with life. This universe was darkened by the hair and welcoming embrace of Shar and illuminated by the cool radiant face of Selûne. However, there was no fire or heat on any of these bodies. Desiring to nurture life on the worlds that formed her body and limbs, Chauntea asked the Two-Faced Goddess for warmth. Then, for the first time, Shar and Selûne were divided, being of two minds on whether they should let there be more life on the worlds or not.

The two goddesses then fought over the fate of their creations. From the residues of these struggles emerged the original deities of magic, war, disease, murder, death, and others. Seizing an advantage, Selûne reached out of the universe altogether and into a plane of fire and, though it burned her painfully, brought forth a fragment of ever-living flame. She ignited a heavenly body—the Sun—in order to give warmth to Chauntea.

This greatly enraged Shar; she renewed her assault on her injured sister and began to blot out all light and warmth in the universe, or the lights of Selûne, gravely weakening her. To protect the early life, Selûne tore out some of her own divine essence, though it nearly killed her, and hurled it at her sister. Selûne's essence tore through Shar, bonding with some of Shar's essence and pulling it loose. This magical energy combined to form the goddess Mystryl, the original goddess of magic. Although Mystryl was composed of both light and dark magic, she initially sided with Selûne, her first mother, giving her the upper hand. Mystryl balanced the conflict and mediated an uneasy truce. Shar was cast into her darkness for centuries, enabling light and warmth to bathe Abeir-Toril and the other worlds.

The battle left Selûne deeply wounded, and thereafter her power would wax and wane with the ages, though she would gain strength from alliances with her daughters and sons, as well as interloper deities from other planes. Meanwhile, Shar, who'd retained much of her might, once again grew strong, and was aided by the shadevari. Consumed with bitterness and loneliness, she vowed revenge and lurked in the darkness until her time to strike. The war between the sisters would go on forever more, but life struggled and flourished on the worlds, watched over by Chauntea.