Said to be the first true wizard’s circle in Khorvaire, the Guild of Moonlight and Whispers was founded in ancient Thaliost by Margana Lain. The guild was dedicated to finding ways to replicating the mystical powers of the fey through arcane science. Members specialize in illusion, enchantment, divination, and magic related directly to the fey, and the Guild is an exceptional source of knowledge regarding fey and archfey.
Moonlight isn’t a large circle, in part because of its extremely high standards. However, it is the most widely respected circle in Aundair, in part because Princess Aundair was herself a member of Moonlight and Whispers. While its members are devoted to their nation, they believe that the Arcane Order of Aureon’s obsession with laws and political power undermines the pure pursuit of arcane knowledge. When someone challenges the Arcane Order of Aureon in the Arcane Congress, it usually takes the support of Moonlight to have a chance to succeed. Breland’s Guild of Starlight and Shadow was founded by members of Moonlight, and the two are generally friendly; while Starlight has less of an interest in the fey, both circles have a common interest in illusion and enchantment. Members of one of these circles will usually be admitted to the halls of the other, though not accorded the privileges of full members.
Mechanically, most members of the circle are wizards, but the guild does accept Archfey warlocks as long as they are proficient in Arcana; the circle is devoted to understanding fey magic, not simply using it.
Source: http://keith-baker.com/ifaq-wizard-circles/