The Silent Brotherhood
The Silent Brotherhood, Dark Brotherhood, or the Eight Swords is an organization of Paladins and Dark Knights who are the followers of Morrighan and are tasked to save the prophesied oracle from any danger until she fulfills her destiny. According to Gwynn, the Royal Army wiped out the Silent Brotherhood, or so they believed.
During the Pact, the player asked Nyle if the Silent Brotherhood can use their magic to cure Tieve from her sleeping illness. Thus, it resulted as the player joining the brotherhood and gaining the Transformation skill.
The Brotherhood is composed of Paladins and Dark Knights, who use extreme powerful abilities on their foes, however this power is not entrusted lightly.
Brynn isn't a Silent Brotherhood member. However, Nyle treats him like the brains of the group, as Brynn has been assisting the Silent Brotherhood.
The Silent Brotherhood is called the "Eight Swords" because there only 8 members. That number went down to 7 when Nauches was killed by the Giant Spider; it is then implied that the player joins the brotherhood, restoring the "Eight Swords". The other 4 members in the brotherhood are unknown.
Nyle mentions Sylvan during the Pact story quests, but it is unknown whether Sylvan is a member of the organization or merely a location.