- “It's magic, I can tell.”
- ―Tara Maclay[src]
Magic (also refered to as magick, mojo or the whammy) was the manipulation of supernatural forces to alter a certain aspect of reality, with effects that defied the conventional laws of nature. Essentially, it was a way to reshape reality on fundamental levels.
Usage and Execution
Magic did not rewrite the laws of physics, but instead worked off of them, adhering to the basic physical laws of nature. Consistent with the Law of Conservation, magical energy could not be created or destroyed, but transferred with the help of a catalyst. All spells left a trace signature after they were cast which weren't perceptible to the human eye unless the ritual Tirer La Couverture was preformed.
- Potions: Substances created from the mixing of certain ingredients.
- Spells: Recitation of text which summoned a particular force: demons, spirits or deities, in order to affect reality in a determined way.
- Talismans: Physical objects in which certain abilities had been imbued.
- Rituals: A combination of physical actions, spell recitation and use of talismans, potions and other ingredients to manipulate reality. Sometimes they also required specific times and locations.
PotencyWith proper training, magic practitioners become capable of performing feats of magic through sheer will. These acts ranged from minor telekinesis to more complicated feats of transmogrification, levitation, and summoning of energy blasts or lightning bolts. Some people, Willow Rosenberg for example, had a greater inherent talent for magic than others. There also seemed to be a commonality between magical power and family heritage, particularly on the maternal side. For instance, both Tara Maclay and Amy Madison were powerful witches and both of them had mothers who were also adept at using magic. Magic could also be transferred into a host for a limited time, as was the case with Rupert Giles, who had immense magical power bestowed upon him by a very powerful coven in England who then sent him to Sunnydale in an attempt to stop a rampaging Willow from destroying the world.
Though all living beings, humans and non-humans alike, could perform spells with the usage of artifacts and incantations, witches and warlocks typically had greater knowledge and power over such forces.
Many spells called upon higher beings or named gods in their incantations. Such named Gods included: Chaos,Aradia, Kali, Hera, Kronos, Thonic Thespia, Diana, and Hecate. Other spells called upon demons such as Proserpexa.
Types of Magic
There were several branches of magic that are disciplined to and focus on specific subjects.
- Necromancy was a form of magic that involved the manipulation of the dead and undead.
- Flesh Magic was an ancient form of magic that relied on flesh and blood rather then words or incantations.
OveruseMagic also had its limits, both physically and mentally. Overuse of magic could lead to addiction which, if left untreated, caused the practitioner to eventually disintegrate or burn out. Since magic required physical energy in order to function, only so much could be used at any one time as posited by Willow Rosenberg in her battle against the hell God Glory and after resurrecting Buffy Summers. Likewise, after a night of heavy magic use, Willow returned home exhausted and unable to use even simple magic to close her blinds. This clearly indicated that overuse of magic and lack of energy had localized detrimental effects on the practitioner's magic use. Magic could also cause mental duress and painful headaches, e.g., after Willow used a very powerful teleportation spell on Glory, she had recurrent headaches for roughly a week or so after. Heavy nose bleeding was also common of magical overuse or after completing a very powerful spell as was the case with Willow Rosenberg a number of times, e.g., creating a large forcefield when the Scooby Gang were on the run from Glory and the Knights of Byzantium and, likewise, when Willow created a forcefield to defend the Summers' home against a Turok-Han. When the Slayers' base was under attack by Twilight's army, a few of the witches protecting the keep had what appeared to be brain aneurysms through severe overuse of magic.
Risk factorMaintaining control while preforming a spell was challenging in itself and required concentration. Willow could levitate a pencil at her will so as long as she retained "emotional control" and upon hearing something to her dislike, she accidentally torpedoed the pencil into a tree. When imprisoned in a frat house under the influence of a demon, Buffy warned Willow, only then a witch practicing since last year, that she shouldn't use magic and her spells were just "50/50". A stubborn Willow proceeded to use magic anyway and the spell backfired. Tara seemed nervous in summoning the goddess Thespia  and she, along with many others expressed hesitation that Willow would be able to pull off a resurrection where the ritual itself was a trial of torture. Although Willow ended up successful, the offering was ruined.
Spells could have unwanted results if things were to go wrong during the performance of one. A love spell that Xander Harris had Amy perform on Cordelia went wrong, resulting in every woman in town falling for him as Cordelia already loved him. Willow tried to alter the memories of Buffy and Tara but ending up removing all the memories from them, herself, Xander, Rupert Giles, Spike and Dawn Summers, simply because she unknowingly left a bag of Lethe's Bramble to burn, a necessary ingredient.
Physical EffectsMagic could also have manifestly visible and bodily changes on the caster, the most common of which was the caster's eyes turning completely black when using very dark or powerful magic. This was shown in Catherine Madison and her daughter when preforming feats of transmogrification.
Willow was subject to severe physical change after she absorbed the latent magical power confined within dozens of spell books, which turned her hair and eyes black and caused dark veins to sprout up on her face. After this point, whenever Willow over exerted herself, her hair and eyes turned black and her voice often deepened and had a slight echo to it. Willow used the power of the Slayer Scythe to turn every potential in the world into the real Slayer. This temporarily caused Willow's hair to turn white and her whole body to glow with a godly light. After Tara was shot, Willow's eyes turned blood red.
The End of Magic
Early in the 21st century, Buffy destroyed the Seed of Wonder. The doors to other dimensions were closed off from the earth, meaning that witches and warlocks could no longer draw on magic from demons, gods or other realities. While magical beings, such as Slayers, demons, and vampires, already present in this reality continued to exist, the Slayer line was rendered dormant and all vampires sired after the Seed's destruction were mindless. Earth was essentially without magic. In a dream, Buffy discovered that the Slayer Scythe was the key to restoring magic to the world. After regaining her own magic, a new Seed of Wonder was spawned directly Willow Rosenberg's body, rendering her powerless in the process.
By the 23rd century, magic began to return to the world.