Karsus

When Karsus stole Mystryl's power, many believe he miscast the spell and destroyed the goddess and the weave. However, Mystryl was unwilling to give up her power and created a zone of wild magic around Karsus as he cast the spell. Wild magic conveys a high chance of miscasting, but also a small chance of success with empowered results. This is what happened. Instead of temporarily taking Mystryl's power, he took it permanently. Still encased in a zone of wild magic, he took over the weave and the wild magic spread like a disease, infecting eventually every part of the weave. Karsus, now the first Mystra, spent the next decade repairing the weave to the best of his ability. There are still regions of wild magic to this day that remain unrepaired.
Karsus, Mystra, died during the time of troubles when the gods walked the earth. He attempted to once again climb the steps into godhood and was struck down by Helm. The weave was already in disrepair as there was already no god of magic. Midnight, a young sorceress, later took his place and upon ascension to godhood, repaired the weave once more. She remained the new Mystra until Cyric assassinated her with the help of Shar.