Mystra

A greater goddess of the Faerûnian pantheon and the second incarnation of the goddess of magic after her predecessor Mystryl sacrificed herself to protect the Weave from Karsus's Folly. The Mother of all Magic, she was destroyed by Helm when she defied the will of Ao the overgod and attempted to leave the Prime Material Plane during the Time of Troubles. At the conclusion of the Godswar, Ao offered the position of Goddess of Magic to the wizard Midnight, who reluctantly accepted and took Mystra's name in order to smooth the transition after so much chaos.
The only known description of the mortal that became Mystra was of a "beautiful peasant girl" who was just beginning her studies of the Art. As a goddess, Mystra could shape change at will and mortals who saw her reported she changed constantly. Others described her appearing in a form resembling a multicolored will-o'-wisp. When Mystra revealed herself to Midnight just before the gods were forced to walk the land of Toril, she was at first just a dark figure surrounded by an aura of blue-white light. As she created the pendant that held some of her power, the glow revealed the face of a "beautiful, willful young woman, with hints of a long-borne pain." Mystra's avatar at the time was a slim, young girl named Caitlan Moonsong who had a graceful bearing, large, dark eyes and short-cropped blond hair. Caitlan went barefoot and wore plain ankle-length robes of a dark hue. When possessed by Mystra, she appeared taller and her hair moved as if it were alive.
This incarnation of the Mother of All Magic was quite the opposite of her predecessor, Mystryl. Whereas Mystryl was quixotic and quick to change, Mystra was said to be a manifestation of the Cosmic Balance—maintaining order in the world of magic by correcting great inequalities if things went askew. Indeed, in the wake of the Fall of Netheril, one of her first acts as the Lady of Spells was to ban the use of all magic greater than ninth level. Among sages and historians this was known as Mystra's Ban. She was not as concerned with the ethics of Good versus Evil as long as the laws of magic were upheld. Early in Mystra's tenure, she chose a champion, called the Magister, to wander the Realms, applying the Art as he or she saw fit, choosing enemies that would bring glory to the title. She fully expected that her representative would be challenged and eventually defeated by other mages, only to be replaced by another spellcaster worthy of the office. It was her belief that in this way the Art would be strengthened and improved through conflict and adversity.
Toward the end of her reign as the keeper of the Weave, she had grown tired and bitter, fed up with the schemes, maneuverings, and blatant attacks that her preeminent position attracted from every power-hungry being, sycophant, and fawning parasite, for century after century. Her pride in her status, and the kindness and respect she received from those that aided or worshiped her for other than magical gain, kept her hopes alive.
The Lady of Spells could cast any spell ever known at maximum level, one offensive and one defensive spell per minute. The only exceptions to this were if she cast wish, time stop, alter reality, or gate, which required enough concentration that no other spell could be cast in combination. She could shape change at will and could grant other creatures spell knowledge by touching them. She controlled and provided the Weave, allowing (relatively) safe access to the power of raw magic to mortal spellcasters and magical craftsmen.
The first Magister was Azuth, a powerful and brash young wizard who found joy and beauty in works of Art. Eventually, she sponsored him to godhood to serve her and he became a demipower, the Patron of Wizards, passing the mantle of Magister on to his successor. Another strong wizard, this one efficient and methodical, caught Mystra's eye and she also sponsored Savras the All-Seeing to godhood. Azuth and Savras both sought the favor of Mystra (some say romantically as well) and the two fought for many years, both gaining in power, becoming lesser deities in the process. Eventually, Azuth defeated Savras, reducing him back to demigod status and imprisoning him in a jewel-encrusted staff, the Scepter of Savras. Azuth grew to be Mystra's friend and adviser and they had great affection for each other. Mystra also had powerful unaging servants among her ranks of followers, including Elminster, Khelben Arunsun, the Seven Sisters, the necromancer Sammaster, and Gale Dekarios.
All manner of spellcasters (clerics, crusaders, druids, monks, paladins, rangers, shamans, and wizards) were included in the hierarchy of Mystra's church. Wizards who primarily followed Azuth or other deities would usually show some devotion to Mystra as well.
Moments after the Fall of Netheril, a peasant girl with rudimentary magical training but an aptitude for spells became the new Goddess of Magic and managed to catch three of Netheril's floating cities as they plummeted, bringing them to rest on the ground and saving thousands of her people. Being of a more lawful disposition than her predecessor, she laid down new rules for all spellcasters, banning spells above 10th level, limiting who had access to magic, limiting how many spells of each level could be held in the mind, increasing the time it took to cast spells, and forcing long periods of memorization or prayer each day to replace spells cast previously. Arcanists no longer had to specialize in a form of magic but could generalize. This eventually led to arcanists being called wizards.