I think the operative word is 'secret'. Sure those small-scale petitioners may be "combinations" that are misguided, possibly evil, but they're obviously not secret, their goals are in fact the opposite. The best example I can think of would be Yale's Skull and Bones. I suppose if anything out of one of Dan Brown's annoyingly ridiculous books were true, that would qualify -- even though the secrecy is eventually unveiled, secrecy was required for at least a large part of the org's lifetime to accomplish its goals.

I'd say the US's love triangle of corporations / lobbyinsts / congress is about the most public a secret combination can get. We publicly know there is corruption, but we don't know the actual deals being made or the true extent of the corruption. Kind of like the Gadianton robbers. Obviously there's some secret combinations in the military-industrial complex as well but nothing else comes to mind. OP are you thinking of something in particular?