Arcane magic

Arcane magic (also called the Art) was a form of magic involving the direct manipulation of energy. Practitioners of arcane magic were generally called arcane spellcasters or arcanists.

Nature of Arcane Magic
It differed from other types of magic such as divine magic because the wielder was tapping directly into the Weave itself.

Arcane magic was created out of manipulation of the Weave. Anyone who could fully duplicate the gestures and sounds required of each spell could use arcane magic. Manipulating the Weave could be done via magical writings, rune magic (though rune magic was more involved with divine magic), gem magic, circle magic, etc.

Historically, arcane magic was the most unreliable form of magic.

Creation of the Weave
The original deity of magic, Mystryl, was created from the energy produced in the battle between the deities Selûne and Shar, and was key in providing a balance in this ongoing conflict. Her birth had the effect of creating the Weave, a sort of ethereal layer that pervaded all of space. It was by drawing upon the Weave that mortal creatures were able to use magic.

Mystryl and the Weave could not exist without one another, because she was the "embodiment" of the Weave itself. She acted to repair the Weave, and where action occurred that harmed the Weave, Mystryl was harmed as well.

Karsus's Folly
In, a Netherese arcanist named Karsus prepared and cast a spell called Karsus's Avatar, which proved too great in power for him to control, and caused a massive failure in the Weave. At this time, there was no practical limit to the power of arcane magic or the spells that mortals could use.

Mystryl chose to sacrifice herself to save the Weave, a choice that led to her death, and the subsequent rebirth as Mystra, who was able to repair the Weave, although Netheril was destroyed.

Mystra's Ban
To prevent a similar episode occurring again, Mystra imposed restrictions on magic that used the Weave. Firstly, she banned a number of very high-level spells that she considered too powerful for mortals to use responsibly. Unless the wielder was able to harness epic magic, the limit could not be circumvented. Casters were also limited in the number of spells they could keep in their heads at any one time, therefore further reducing their capability to damage the Weave.

The Magister
In an attempt to further control the Weave, Mystra (the ascended Midnight) appointed a Magister in the form of Azuth, a mortal mage who became deity, and became responsible for the championing of magic.

Time of Troubles
In 1358 DR, during the Time of Troubles, Mystra, in corporeal form, was slain in Waterdeep during a battle against Myrkul and other beings of great power. This caused her "essence" to merge with the land, and subsequently, wild magic occurred in some areas, while the Weave failed completely in others.

A mortal wizard named Ariel Manx, and dubbed "Midnight", took on the mantle of divinity and portfolio of Mystra. She was able to mostly restore the Weave and return magic to normal, and her focus leaned more towards the proper use of magic instead of simply maintaining a balance.

As the new Mystra, Midnight continued to respect those who sought divine magic from her. Midnight's slight leaning towards good became evident, but she did not discriminate, and from her ascendancy onward, she would allow any arcane spellcaster to access the Weave.