Mystra

Goddess of Magic

To understand Mystra is to understand the history of four goddesses. Mystryl was the first goddess of magic, created when Selûne and Shar were fighting and Selûne threw some of her very soul through her sister. Mystryl died in -339 DR as she sacrificed herself to save the world from the chaotic magic released by Karsus. She reformed herself as Mysta.
During the Time of Troubles, when the overgod Ao made all the gods mortal to make them care about their worshipers, Mystra considered herself above that and tried to return to the outer planes. Helm, the only god to retain his powers and set to prevent the gods from returning, killed her for trying. Instead a mortal woman ascended, calling herself Midnight.
In the Year of Blue Fire 1385 DR, Cyric, encouraged and helped by Shar, murdered Midnight in an attempt for Shar to take control of The Weave. What actually happened was the Spellplague as the weave tore asunder in various ways. By 1479 DR the essence of Mystra had merged with that of Midnight and she returned once again.

Worshipers

The church of Mystra preserve magical lore so that magic will continue and flourish in the future even if the dominant races of Faerûn were to fall. Its members also seek out those skilled in magic or who had the potential to use it, keeping a close eye on those who were likely to become skilled. Her clerics are encouraged to explore magical theory and create new spells and magic items. Sites dedicated to the goddess are enhanced by the Weave to allow any spell cast by her clerics while in them to be affected by metamagic. Mystra honour commitments to members of her predecessor's clergy who'd joined the church before the Time of Troubles, preventing them from being forced to leave the clergy due to alignment differences.

Mystra's Chosen

A lot of Mystra's power is bound in mortal beings known as her chosen. Elminster Aumar and Khelben Arunsun are known to have been among them, but several more are presumably out there. For instance, some say Laeral Silverhand is one.

Divine Domains

Magic

Mental characteristics

Personal history

The third incarnation of Mystra started as a human wizard born in 1332 named Ariel Manx, but often going by her nickname Midnight. She became a worshiper of (the second) Mystra as she began a life as an adventurer. When all the gods were sent to live as mortals on the prime material plane during the Time of Troubles, she and her group took it upon themselves to find the Tablets of Fate that could restore order. During this time, the previous Mystra was killed by Helm as she tried to ascend to the outer planes.
Midnight eventually managed to succeed in her mission and Ao chose her to take the position of Mystra. Meanwhile, two of her previous adventuring friends also godhood to one degree or another: Kelemvor and Cyric. Since the latter reached his position by taking over the portfolios of deception and murder, his eventual betrayal might not be that surprising.
Mystra doesn't talk about what happened to her after Cyric killed her, but it's apparent that this new goddess of magic is Midnight at her core, but somehow merged with the old Mystra and those who remember even claim there are traces of Mystryl in there.

Morality & Philosophy

Mystryl was quite fickle and always changing how she viewed the world, and magic behaved accordingly. The Mystra who took over was less inclined to side with good over evil than her predecessor (which weakened Selûne during her existence), so long as the laws of magic were upheld. She was the result of the previous incarnation having had to kill herself because a mortal magic user went out of control so controlling The Weave more tightly became important.
Midnight, being a mortal, had an approch that was slightly different as she tried to do good while still keeping the balance to some degree. The returned Mystra after the Spellplague has a more neutral attitude, but still muck more allied with the good gods than the evil ones.

Social

Contacts & Relations

Mystra's customary adviser is Azuth but she is also served indirectly by Savras and Velsharoon. Other allies of hers include Selûne and Kelemvor, whom she knew as a man when she was a mortal. Mystra's greatest enemies are Shar, who created the Shadow Weave to oppose Mystra's Weave, and Cyric. Her enmity with those, in particular, has not been helped by the fact that they killed her...
To a lesser extent, Mystra works together with Oghma.

Divine Classification

Greater Deity

Honorary & Occupational Titles

The Lady of Mysteries
The Mother of All Magic
The Lost Goddess