You can't study witches and witch lore without running into Aradia.
Conceived or discovered by (depending on your point of view) Charles Godfrey Leland, Aradia is something of a witch Messiah. A promised one to lead witches (and pagans in the old sense of the word) to understanding. Her coming was detailed in Aradia or the Gospel of the Witches, which you can read as a scanned image or as HTML text.
Aradia is a wholly interesting figure. The daughter (reportedly) of Lucifer and the goddess Diana she was raised as human, was the first witch who taught others the secrets of witchcraft. She rose to be the Queen of Witches and then went on to become something akin to a Goddess herself.
Various Witchcraft Traditions even use Aradia as the name for their Goddess or even as the Moon Goddess.
When I began my own research back in the 80s the discovery of Aradia was very interesting to me. It seemed like there was some sort of link between pagan practices of the modern times (including the sensationalism of the 70s occult revival) and ancient times. But it didn't take me too long to start picking apart the arguments and see that the scholarship here is not really up to scrutiny.
But that doesn't mean I can't use it for my own worlds!
Here she is for the Pathfinder RPG.
Titles: The Goddess of Witches, The Queen of Witches
Portfolio: Magic, Women
Worshipers: Witches, pagans
LG NG CG
LN N CN
LE NE CE
Domains: Magic, Knowledge, Protection, Moon
Favored Weapon: Quarterstaff
Aradia is the goddess of witches, witchcraft, the moon and magic. To a lesser degree she is also the godess of hags, fae and other supernatural creatures.
Aradia is believed to be the daughter of the Goddess of the Hunt and Moon and the Prince of Devils. She lived, not as a Goddess, but as a human for her life and was a teacher to the first witches. She later became the first Queen of Witches and held the first witch courts.
Aradia is often considered to be a pretender god or worse a false goddess. Her time spent on among humans has given her a "taint" among the other gods. She gets along favorably enough other gods of magic but her best relationships are with gods and goddesses of the harvest, growing and natural cycles. She is also on very friendly terms with gods and goddesses of the moon.
Her dogma also makes her appear less in the eyes of the other gods. She focuses primarily on her witches and any that reach to her. She is disliked by any god or goddess with very strong ethos (LG, CG, LE, CE). Gods of Sun, Law, Justice also tend to dislike Aradia.
Appearance and Emissaries
Aradia appears as a peasant woman in simple garb. Her facial appearance fluctuates so one is never sure if she is a young or old woman. In all of her forms Aradia appears wearing a crown with a crescent moon horns turned upward.
Herodias - a human servant of Aradia. She was Aradia's first student and some claim she could even be her own daughter. The number of stories of the two are so confounded that one is often mistaken for the other.
Holda/Huldra/Frau Holt - Holda is a the Patron of all Hags. She is less their Goddess and more the most powerful representative of their race. She appears as a frightening woman or hag, nude and riding a giant goose.
Nicnevin - Queen of the Faerie Witches. She appears as a stately Elven woman in regal dress bearing a crown. She is the patron of Elvish Witches (Kuruni), faeries and nymphs.
Church of Aradia
Aradia has no formal religion outside that of her witches.
Worshipers and Clergy
Aradia is worshiped primarily by witches but also by like minded clerics and wizards. A number of folk magic practitioners also also count themselves among her worshippers.
Temples and Shrines
Temples of Aradia are commonly known as "Sky-thatched temples" in that they are held outside under the sky in natural settings. Worshippers congregate "skyclad" or while nude.
As witches the worshippers of Aradia hold their own Books of Shadows as their holy texts, but a few are are considered to be great scholarly works of Aradia.
The Gospel of the Witches - The primary text of stories and practices of Aradia. This book is 1/3 historical prose and 2/3 spell book. The book is a collection of the writings of a number of different authors handed down through the years, presumably starting with Aradia herself.
The Vangelo - Possibly an older version of The Gospel of the Witches, the Vangelo (the Gospels) are more liturgical in nature. The differences in these books are minor, save for age, but are points of contention among the different sects.
As witches the followers of Aradia celebrate on the 8 Sabbat days of the year; the 4 seasonal days (Autumn Equinox, Winter Solstice, Spring Equinox, Summer Solstice) and the 4 cross-quarter days (Samhain, Imbolc, Beltane, Lammas).
Witches of Aradia also celebrate the phases of the moon, the crescent moon in particular and the crescent moon just before dawn and just after sunset the most of all.