This material is offered in elaboration on the basic idea of the Sisterhood of Lenience.
Many of the trappings of the night hag are what Americans generally think of as relating to witches. We may wish to consider building up a class that evokes those connections; there is no straightforward "witch" character class, though there are some specific types. In particular, constructing a class around Unearthed Arcana's concept of incantations -- http://www.systemreferencedocuments.org/35/sovelior_sage/unearthedIncant... -- nicely models the weird ability of hags (and thus, their students') predilection for achieving substantial magical effects apparently through study of eldritch lore and unique constructions rather than standard spell lists. It also explains why hags are so interested in magical research: incantations are constructed for specific purposes, and need a lot of specific information to design. In a sense, they're "assembly-level" magic, as opposed to the "high-level language" of regular arcane magic, or the ultimate "DWIM algorithm" of a wish.
This might be an NPC class or a PC class. Before I go through all the effort of constructing a Witch class for the Darkest Gray project, your comments on the usefulness of such a class would be appreciated. Here are some of my basic ideas:
Hit die, armor, shield, BAB, saves, skill points: as wizard.
Weapon: One simple weapon (as commoner).
Equipment and treasure: As wizard. A book of incantations is possible but hardly necessary -- incantations are skill-based, and can be learned by rote. For that matter, many witches may be illiterate.
Class skills: Knowledge (Arcana) is vital for incantations and Bluff is good for those who have to hide their identity. Concentration and certain languages are useful for the rituals themselves. Knowledge (the Planes) and Spellcraft may be useful for researching new incantations. Craft, Handle Animal, Profession, and Ride would all not only be part of the knowledge base of the commoner population the Sisterhood draws from, they could be useful if advanced witches get opportunities to summon nightmares.
Feats: Summon Familiar. Skill Focus (K(Arcana)). Later bonus feats to defray or foist off the costs of backlash or failure on an incantation, modeled after the XP transfer methods described here (a free WotC web enhancement, though as I post there is another object there). Some of these may only be useful to those of evil alignment, and/or those neutral on the law-chaos axis. A list of available variants of Brew Potion and Craft Wand for incantations. Ability to rapidly research a new incantation for a specific situation.
In particular, I rather like "Fundamental casting: the magical training offered to a witch is of a primitive, inefficient type, but employs concepts that underlie magic in general. Her witch levels stack with levels she later takes in any spellcasting PC class to determine caster level for that class, whether divine, arcane, or other. They grant no other benefit of that class, such as number of spell slots or level of spells known."
Incantations: a list of these, with a starting witch knowing a certain number and able to obtain more whenever she can study one (probably a K(Arcana) check) or receives one in training upon gaining levels. Intricate and slow, with unpleasant backlashes and difficult material or XP costs for most, in addition to being very specific in their purpose. Yet colorful and evocative, in the style of Cook's Unearthed Arcana, drawing from real-world hedge superstitions and the ravings of our world's more fanatical witch-hunters. Broomsticks, yarrow stalks, flying ointment! Poisoned apples! Trysts with demons at midnight! Hexing the cow, poxing your rival in love, and giving that lecherous old Mayor the Evil Eye! Snatching babies, cooking up larvae, eye of newt and wing of bat! How about an incantation that requires the witch to eat her own familiar? Cackling, lots of cackling!
Hmmm. Interesting. I like the idea of using incantations to represent the subtle magics of a Hag. If you're interested in more ideas for possible magics to be worked, I have a copy of Jacobus Sprenger and Heinrich Kramer's Malleus Maleficarum lying around here that i could re-read with the intent of finding some things. As the handbook of the Inquisition it has quite a list of accusations, but it'll take a bit of digging for me to get them in a reasonable order.