I believe it has been established that The Circle and the Black Council are the same thing. The Black Council is the name Harry came up for them, but I believe The Circle is what they call themselves. I think we learned that in White Night from Cowl, but don't hold me to it.
I think we only assume so, at this point; and you know what Harry likes to say about you and me...
I've seen a cogent argument that "the Circle" doesn't even exist -- that it's a figment fed to the credulous by Cowl.
Nothing has explicitly linked Peabody to any other operator; since he was competent enough to work undetected for so long, he could credibly be competent enough to be a "lone wolf," resentfully turned to black magic to become "the power behind the throne" since he was so despised (sometimes cordially) by the powers-that-be ON the throne.
Thorned Namshiel is only assumed to be the source of the Hellfire at Arctis Tor (though Mab semi-confirms it, by not denying it; but... FAERIES, god dammit...).
Furthermore, T.N. (or whoever was (or were, there may not have been just
one Denarian; or
any Denarian(s) -- Harry had Hellfire; maybe some other mortal wizard brought Hellfire to Arctis Tor) there) is only assumed to be collaborating with a larger Circle or Black Council.
The Hexenwulf belts, the overpowered stuff given to Sells & Kravos...
It's really all circumstantial.
One of the things we've heard repeatedly is that the Outsiders
all work together, to their own singular agenda. That kind of distributed-but-unified operation looks an awful lot like Harry's theoretical "Black Council..." might in fact
be the only organized Black Council there is.