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 (Elysium or Mechanus)
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 wekl 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.