The first mention I believe was in "the North" set, on page 58 of the sourcebook. Before (and after) that refer to lore about the Stronghold of the Nine. At that point it was just the name of their location.
In Lost Empires it became the name of the leaders council and is mentioned several times - most importantly a 3 or 4 paragraph description.
Also (I think) it is mentioned in Return of the Archwizards when he goes to visit his mother who is the lady of the wood and in The Last Mythal series when they are moving through the High Forest.
If it has ever been mentioned otherwise I do not know of it.