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The source of arcane magic is the ley lines of a planet, which can be best described as being analogous to the blood veins in a person's body, only containing magic instead of blood. When Azeroth was reforged by the Titans, they created the Well of Eternity, an enormous font of magical energy, to say the least. When the Sundering tore apart the world and destroyed the well, its arcane power spread throughout the world, giving rise to the ley lines of today.
The Kirin Tor have divided arcane magic into seven categories:
Abjuration, representing protective magic, such as armor spells;
Conjuration, the creation of creatures and objects, everything from cookies to elementals;
Divination, spells dedicated to gathering information, such as revealing the invisible, or seeing across great distances;
Enchantment, imbuing an object, or person, with magical power;
Illusion, including spells that can turn the mage invisible, or manipulate foes;
Necromancy, magic involving raising and controling the dead, considered to be illegal;
and Transmutation, the manipulation of time and space, with spells such as Polymorph or Portal spells.
Depending on who you ask, the number of schools can jump up to eight or nine, including Evocation. Evocation is considered to be the school covering the gathering magical energy or focusing one's mind.
There is a set of rules governing the use of arcane magic. These are as follows:
1. Magic is powerful.
2. Magic is corrupting.
3. Magic is addicting.
4. Magic draws the denizens of the Twisting Nether to its users, also known as demons.
In addition, while magic can greatly extend a user's life, best exemplified by the Guardian Aegwynn living more than a thousand years, the stress of magical energies can prematurely cause a wielder's hair to turn gray.
Long ago, the planet K'aresh was invaded by the Voidlord Dimensius the All-Devouring. The creature opened countless gateways into the Twisting Nether and the Void, bathing the world in arcane and dark energies. The only known survivors of that world are the ethereals, and the massive exposure tore away their corporeal bodies, forcing them to bind their magic-infused souls with enchanted strips of cloth to survive, while also enhancing their minds and magical abilities. The event is similar to the destruction of Draenor, which, while obviously less severe than K'aresh, still managed to twist some of its inhabitants, the most notable of which are the Nether Dragons.
In the words of Medivh, "When someone handles an item, they leave a part of their own magical aura attached to it.". This is known as the Law of Sympathy. The sympathy between someone and the object used by them becomes stronger with more constant and thoughtful use. As magical auras vary with each person, a mage can study the objects handled by someone to learn various amounts of information. Experts can even tell the contents of handwritten letters or books without having to actually read them.
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