Andigen, tales from the frontier
The sisterhood is a lose knit secret group of evil female bards. They form a kind of underground spy and mutual support network. Think of a kind of evil females only Freemasons.
Due to their secretive nature, few have ever heard of the sisterhood, and of those, most think it just a myth. But the sisterhood traffic information all over The Land, and can be consulted for information at a hefty price. Retaining the sisterhood to actively investigate something or someone costs even more. They do not assassinate or hunt people. They are not thieves, although many have levels in rogue. Predominately, they are pure bard. Often they are hired to help ensure a favourable outcome for somebody with a particular political agenda.
If the sisterhood as a whole has an agenda, it is a closely guarded secret known only to the grand masters, of which there are always only three. Often they can be found covertly affecting the outcomes of political intrigues to their own ends, or for profit because they were paid to do so.
Members of the sisterhood are frequently not traditional bards. They don’t hang around in bars strumming their lutes. While some indeed play instruments as part of their performance, many are also dancers and courtesans (think Geisha).