An old herbal tradition from the Green Fields
"The Grannies" are part of an old magical/herbal tradition from Green Fields, continued in the agricultural fragments outside The City, where old (and sometimes not so old) women share folk remedies and natural secrets while growing special herbs in their gardens. Some of these "secrets" are common to all women-folk of the Greenfield farmers, but some are kept only by the Grannies.
The remedies of the Grannies are simple: a tonic for back pain, a tea for fertility, a charm to keep rabbits out of the garden. Nothing flashy that the arcane academics of The City have been able to latch on to. Though, rumors that the Grannies meet in the corn fields during the summer solstice to perform some kind of grand (or nefarious) ritual has existed since before they joined The City.
The Grannies are mostly halflings due to their origin from the Green Fields, but in the decades since they've joined The City, intermixing of the ancestries has included a handful of humans and other races in their ranks.