A coven is a group of witches who work together to perform witchcraft.
One notable coven is Morrolan's Circle of Witches, who work in Castle Black.
A standard coven of witches on Dragaera apparently numbers seventeen. A larger coven may number 289 (seventeen times seventeen) which allows far more powerful spells than a single coven.
In the Eastern Kingdoms, especially Fenario, covens are often secretive to prevent animosity from the local peasant populations.
One such coven existed in Burz, before the arrival of Vlad Taltos, who contributed to its destruction.