The Silver Conclave is an island in LotE where Lugia dwells, later revealed as the original single island that composed the Whirl Islands.
In Legend of the Exile, the Silver Conclave is associated with the origin of all Skarmories, hidden from the rest of the world. Shaymin and Jirachi convince Lugia to unconceal the island and summon Skarmories from the world, who carry Pokemon fleeing from Giratina's war. Among these refugees are Slick's Pokemon.
Unfortunately, Mewtwo later raids the island to capture Jirachi, massacring most of the refugees.
Prior to the implementation of custom chatrooms, members used Chatzy as overflow rooms when the Route 50 chat became too packed. Several Chatzy rooms were used over the years, but a frequently used one was Sparkleaf's chatzy, the Silver Conclave.
As the chat tended to migrate to Chatzy, the Silver Conclave was often perceived as an exclusive club by outsiders. The chatzy room was later abandoned, due to Sparkleaf shifting away from using Chatzy, and the Silver Conclave has since been purged from the Chatzy servers. However, Sparkleaf's custom chatroom is #skarmory, a reference to the Silver Conclave in LotE.