Witches & Covens
Though High Magic is considered by most to be true art, it is by no means the only. There are many ways magic manifests in the world - from the natural power of dragons to the complex formulae and processes of alchemy. The term "witchcraft" is often used as a perjorative between wizards - it is akin to calling one a charlatan or a small-time peddler of petty magics. Nevertheless, witchcraft, sometimes called hedge magic, is very real, and encompasses a broad range of magical traditions.
Witchcraft can be very powerful but is rarely flashy or violent as High Magic so often is. It does not produce fireballs or conjure up bolts of lightning, but it is a far subtler craft. In its own way, a skilled witch can be as terrifying as any wizard, for their works are slow and silent, often very difficult to detect. People have a healthy respect for the power of wizards, but a witch is universally feared.
Traditions
Besides the various crafts that make up the practice of witchcraft itself, a coven needs one thing: a source of power. There is a reason, after all, that witches form covens - while alone they are weak, their strength is multiplied by many times when they gather in covens. This power always has a source or comes at a price, however - and there are as many sources as there are covens themselves.
Sacrificial Covens renew their power by blood sacrifice. When a working of power must be done, elaborate sacrificial rituals are enacted and blood flows freely. The vitality of their subjects gives potency to their workings - sometimes a black rooster or a newborn lamb, but of course the most powerful magics will always require human sacrifice. Often, a specific spell requires a specific kind of sacrifice - a virgin, one who has spilt blood without remorse, a person who has been touched by the fairies, and so on.
Ancestral Covens can be some of the more powerful organisations of witches out there, but they are conversely more vulnerable. Ancestral witches draw power from sacred burial grounds - when one of them dies, their body is consecrated and added to the burial grounds, and their magic makes every member stronger. However, this brings with it a price: because this additional power flows from the earth, it diminishes over time. Every witch that dies and is not consecrated effectively weakens the coven - especially if they were one of the more powerful members. Furthermore, they are uniquely vulnerable to attack on their sacred sites: desecration of their ancestors is simple and can drastically weaken or even destroy them.
The Art
One of the things that makes witchcraft so terrifying is its lack of clear limitations. It is a subtle and difficult art, one that is dependent on wisdom and intuition rather than the rigorous mental calisthenics of High Magic. Witches do keep spellbooks after a fashion, but most witches learn the Craft from other witches, their knowledge and skill passed down via oral traditions rather than written ones. It is possible for a great deal of harm to be done by an inept or inexperienced witch with access to a powerful spellbook. It is difficult to define witchcraft because it does not have a simple definition. It is a broad term used to apply to a wide variety of magical traditions. Nevertheless, there are certain themes and types of magic which run throughout the broad multitude of paths that make up what witches know as the Craft.
Alchemy is extremely common in witchcraft, ubiquitous even. Many of the most powerful and brilliant alchemical recipes exist not in the musty tomes of an alchemist, but tucked away in some ancient and yellowed witch's tome passed down from generation to generation. The creation of potions and alchemical concoctions make up the lion's share of magical workings for most witches.
Hedge magic is often used as a derisive term to refer to withcraft as a whole, but it does have a specific meaning. Sometimes called old magic, hedge magic is the use of the most basic and fundamental forms of magic, elements which can be seen even in the workings of the arcane formulae of High Magic. Protection from Evil, amongst the simplest of wizard spells and a staple for any magic dealing with extraplanar forces, draws heavily from hedge magic, for example. The use of the pentagram and other warding circles is one example of hedge magic as used by witches; another is the use of silver, salt and cold iron for the purpose of protection and warding. Most hedge magic is used in conjunction with other forms of witchcraft as a protective measure, shielding the practitioner from the negative effects that using such imprecise methods can wreak. This is not always the case, however; some of the more complex hedge magics blur the lines between witchcraft and wizardry, requiring great mental fortitude, long meditation and rapt concentration in order to cast. In many ways, these more strenuous hedge magics are more similar to High Magic than anything else, poorly formed counterparts to the same spells that a wizard seals in his mind during his morning preparations.
Invocations and Conjurations are the bread and butter of witchcraft. Every powerful working involves these, and it is these that are most dangerous in the hands of those who do not know how to use them properly. Pleas and offerings to extraplanar beings are incredibly common amongst witches; the price can vary wildly. Sometimes it is something small, a gift of honey or milk or the promise of a good deed. Sometimes it is the blood or life of a living creature, the ability to love, memories, or even the immortal soul. Just as there is good and evil magic, there is good and evil witchcraft. Some witches consort with angels, kind fae and benevolent spirits to work good in the world. Others deal with devils and make dark pacts with the servants of evil. A marked difference between the invocations of witchcraft compared to those of High Magic is the manner of calling. A wizard protects himself with ironbound enchantments and commands spirits to appear before him. A witch may be protected by no more than a piece of hedge magic. A pentagram, perhaps, or a simple cleansing of the area to avoid unwanted energies corrupting the working. They may not be protected by anything at all. Knowing this, it is easy to see why this is the most dangerous branch of witchcraft. The witch's familiar is an example of a simple invocation performed by most witches early in their career; a friendly spirit clothed in the form of an animal that guides them in their understanding of the Craft. Note that invocation does not exclusively involve conjuring up an actual entity of some kind; the use of the runes by the Skalds of Nordmaar is in itself a form of invocation.
Empathic magic is not as central to the Craft as the art of invocation, but it is nevertheless a staple of witchcraft. Simply put, it is a form of primal magic that makes use of the forces of magic that connect all things. It is amongst the most subtle, complex and difficult of arts to master, and is used skilfully by a very small number of witches, but when properly employed it can be immensely powerful. The most recognisable and chilling example of empathic magic is the voodoo doll: a poppet connected to another being by the forces of magic, such that whatever happens to one is carried over to the other.
Sorcery, also known as Wild Magic, has its place in witchcraft. This is not surprising; after all, sorcery is the raw expression of the same forces that High Magic employs, the most primitive form of spellcasting. Even more so than empathic magic, sorcery is a difficult and imprecise art to master. It involves tapping into the very energies of the universe, not bound by the words of power and strictures that bind High Magic and make it safe. Sorcery appears in natural forms throughout humans; power of Extrasensory Perception, the innate abilities of mediums and mentalists, and those blessed with the Second Sight are all benefiting from gifts of innate sorcery. Many witches work hard to cultivate forms of sorcery; one common example that has caught the imagination of the common folk is that of the Evil Eye, a form of sorcery where a witch develops the ability to instill negative energy upon their victim with nothing more than a highly charged look, instilling them with weakness, a curse of misfortune or some other ailment through the power of their will. Most witches will cultivate the Second Sight at the very least, allowing them to act as mediators between the material and spiritual realms.