Quote from the story:
"The Venatori Umbrorum were a secret society of their own, steeped in the occult, quietly working against the supernatural forces that threatened humanity. "
"the Thule Society was more or less a long-time foe, using their resources to attempt to use supernatural powers to their own benefit"
Kemmler, and Grevane were members of this Thule Society. Luccio knew who they were as well, which makes me wonder something. As commander of the Wardens it's almost certain that Eb knows about them too, and yet he lets Harry call them the Black Council and didn't share with him who the Thule Society is... I guess it's possible he believed them destroyed, but seems unlikely. He's too smart to believe they are completely gone, so why not tell Harry? Perhaps he hasn't connected that the Thule Society is the Black Council? Again, seems unlikely he wouldn't make the connection.
Anyways what do you think? Maybe the Black Council is not the Thule Society but it seems likely to me.
Don't know if this is of importance because of the timeline issues:
The Thule Society (/ˈtuːlə/; German: Thule-Gesellschaft), originally the Studiengruppe für germanisches Altertum ("Study Group for Germanic Antiquity"), was a German occultist and völkisch group in Munich right after World War I, named after a mythical northern country from Greek legend.
They are tied to Nazi's and Kemmler was supposed to be involved with the both world wars.