Mystryl

the Lady of Mysteries
She was a greater deity and the first incarnation of the goddess of magic. Later incarnations of the goddess were named "Mystra". Mystryl was most notably known as the being who embodied the Weave, the primary source of magic in the cosmos. She supplied and regulated magical energy to all, making possible spells and magical effects. But her portfolio also included knowledge and energy, invention and creativity, song, time, and the season of spring.
Legend has it that at the dawn of time Lord Ao created the universe, and out of the early chaos came twin deities: Shar of the darkness and Selûne of the light. Together, these two beings created the spheres in the firmament, one of which was the world of Abeir-Toril and its spirit, Chauntea. Chauntea desired new and more abundant life for her world and asked the sisters for light and warmth in order to nurture it. In this, the two beings could not agree. Selûne eventually acted on her own and gave Toril a sun, which Shar immediately sought to put out, thus beginning the battle between light and darkness that raged for eons.
At some point the Dark Goddess got the upper hand and doused many of Selûne's motes of light. In a desperate move, Selûne tore a piece of magical essence from herself and hurled it at her sister. When it hit, Shar also lost a portion of her essence and was cast into the void for centuries. From the blending of dark and light energies, Mystryl was born as a being of pure magic inextricably tied to the Weave. Both of the twins were diminished as a result, with the Moonmaiden's wound being more grave. However, Mystryl was ideologically closer to Selûne, and being created from both light and darkness, Mystryl (and later goddesses of magic) acted as a balancing force between Selûne and the Lady of Loss.
In −339 DR (3520 in Netherese reckoning), the Netherese archwizard Karsus took it upon himself to defeat the phaerimm that were threatening the Netherese empire. He had spent years researching Karsus's avatar, the only 12th-level spell ever created, which was designed to temporarily rob a deity of their power and grant it instead to the spellcaster. Karsus chose to rob Mystryl, and when her godly essence poured into him his mind and body were overwhelmed and he found that he was incapable of wielding such power. Mystryl's hold on the Weave was weakened and it began to unravel. Magical effects doubled in power briefly, then became wild and chaotic. To save the Weave from permanent damage Mystryl chose to sacrifice herself, which broke Karsus's connection, killing him, and stopped all magic for a short time. In the time it took for the Mother of All Magic to reincarnate herself, most of the floating cities of Netheril crashed to the ground.
When the goddess of magic was reborn—this time as Mystra—she used as a vessel a beautiful peasant girl who was just learning cantrips but who had the capability of one day becoming an archwizard. She immediately took control of the Weave and magic returned to Toril. In the aftermath of the Fall of Netheril, the new goddess took a much more lawful view of magic use and laid down some rules, including a decree that the most powerful of spells would fail utterly.