Dweomer is the system of magic used in the Deverry Cycle, which is practiced throughout Annwn. The glossary to Daggerspell describes it "a system of magic aimed at personal enlightenment through harmony with the natural universe in all its planes and manifestations," and notes that the word can also be used to mean magic and sorcerry in general.
According to Katherine Kerr, the dweomer system is loosely based on historical European magical traditions, including Rosicrucianism and Khaballa. In the author's note for The Bristling Wood, Kerr emphasises that dweomer is chiefly a spiritual pursuit, and that effects on the physical world are secondary, even if those effects are quite spectacular.
Practitioners of dweomer, called dweomer-workers, are capable of many supernatural feats. With the aid of minor spirits called Wildfolk, they can control the five classical elements of earth, air, fire, water, and aether. Other abilities include scrying (farseeing), astral projection, and shapeshifting.
Broadly, there are two differing philosophies about dweomer and how it is to be used. Those who follow the dweomer of light view themselves as servants of the "great Light that stands behind the gods," and each dedicates himself or herself to a chosen form of service. Those that follow dark dweomer desire power for its own sake, or as a tool to achieve selfish goals.