Mystra

Mystra
Alignment	Neutral Good (Formerly LN and CN)
Divine Rank	Greater Goddess
Pantheon	Faerun
Portfolio	Magic (all of it)
Domains	Good, Illusion, Knowledge, Magic, Rune, Spell
Home Plane	Dweomerheart (Limbo, Mechanus then Elysium)
Worshippers	Any spellcaster
Favoured Weapon	Shuriken

Mystra is the goddess of magic in the Forgotten Realms, probably the most powerful of the pantheon, when you don't include Ao in the list.

Mystra is probably the most well covered of the FR deities, because she undergoes an extensive rewrite with every edition in order to explain in-universe how and why the rules get changed.

History

Mystryl (1e)

Born from the conflict between Selûne and Shar, the original Mystryl was a Chaotic Neutral entity that was relatively flighty and carefree; A real rainbow chick.

She either created The Weave, embodied it, or was a side effect of its creation, but in any case she was in charge of it. Her attitude towards the use of the weave was "do as you please, but just don't piss me off", which meant that magic wielders could perform spectacular feats that were unheard of in later times.

However, this also opened the weave up to abuse, where one particular spellcaster called Karsus decided to create a 12th level spell that would give himself control over the weave itself.

The spell worked... for all of five minutes. Where he wrestled control over the weave from the goddess, but couldn't actually control it, causing magic all over the world to get screwed up. Destroying his own empire of floating cities, and essentially turning everything back to the stone ages.

Mystryl figured she'd rather die than let some idiot ruin the fundamental forces of magic, so she sacrificed herself to destabilise the weave, and gave birth to her successor Mystra

Mystra (2e)

The next Goddess was an entirely different creature. Immediately imposing rules and restrictions on the weave, preventing those kinds of abuses from ever happening again. Capping spellcasting ability to a maximum of 9th level spells, while changing how magic works.

Furthermore, realising that Mystra is essentially the god of "all power" in an uncharacteristically active move, Ao manipulated events to ensure that Mystra was forced to spread her power around so that she did not become "all powerful" and either upset the balance, or try something stupid like usurp Ao himself. So the second Mystra was also more active and set about spreading her influence around.

She fell in love with a human Wizard called Azuth and sponsored him as the first Magister giving him a humongous power boost and helping him on his journeys through the realms, to the point where he became a god himself, then continuing the tradition of magisters by starting a chain of them.

Then she started appointing Chosen amongst mortal followers, granting them a portion of her divine strength. Though the gifts not as poweful as the Magister outright, there were a lot more of them.

Then she realised that sometimes, all this power just burned through mortals if they weren't already strong enough to wield it, or some of them became corrupted by the power and became tryants. So she decided to have children, and spawned a set of daughters who were imbued with the abilities of her Chosen from birth and raised to appreciate their gifts.

All told, Mystra had about half of her divine power wrapped up between her Chosen, her Magister and her lover Azuth, and even then she was still the most powerful deity in the Pantheon.

So when the Tablets of Fate were stolen from Ao and he kicked all the Gods except Helm out into the mortal realm until somebody owned up to it; Mystra figured she could just barge her way back in.

Unfortunately, powerful mortal avatar of a god she was, she was still no god. So Helm got the upper hand and cut Mystra down in front of an city's worth of audience.

Midnight / Mystra (3e & 5e)

The third incarnation began life as a mortal woman called Midnight who was also a travelling companion of mortal versions of Kelemvor and Cyric as well as cleric of Sune called Adon.

Midnight was pivotal in the recovery of the Tablets and was rewarded with Mystra's portfolio, creating a new goddess of magic, but taking the old name.