Arcane magic

Arcane magic (the Art) is the direct manipulation of energy. It differs from other types of magic such as divine magic because the wielder is tapping directly into the Weave itself.

On arcane magic, Khelben Arunsun said:

"You think that magic is a tool, like a hammer, something that you pick up when you need it, swing it around for a while, and put down again when you're done with it. Not so. Magic is a living thing—part of the Lady of Mysteries, a deity to whom you only pay lip service. You can't just grab onto her when you need magic, squeeze the power out of her, and discard her when you're done. That sort of treatment comes back to you."

Arcane magic is created out of manipulation of the Weave. Anyone who can fully duplicate the gestures and sounds required of each spell can use arcane magic. Manipulating the Weave can be done via magical writings, Rune magic (though Rune magic is more involved with divine magic), Gem magic, Circle magic, etc.

Historically, arcane magic is 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 pervades all 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 -339 DR, 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 which 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, almost all spells were limited to "9th level" and below, to avoid their power damaging the Weave. 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.

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 took on more of a leaning towards the proper use of magic instead of simply maintaining a balance.

As Mystra, Midnight continued to respect those who seek 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.

In an attempt to further control the Weave and prevent another occurrence like the Time of Troubles, 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.