The reference to the "usual cabal" appears on page 237, chapter of Robertson Davies' Fifth Business. The quote is as follows:
He was killed by the usual cabal: by himself, first of all; by the woman he knew; by the woman he did not know; by the man who granted his inmost wish; and by the inevitable fifth, who was the keeper of his conscience and keeper of the stone.
In order to identify the five people of the cabal, one must use the hints given by the text. First, Boy was responsible for his death. Second, "the woman he knew," was Denyse Hornick (his wife). Third, "the woman he did not know," is Leisl (who knew Dunstan but not Boy). Fourth, "the man who granted his wishes," is Magnus (who shows Boy who he really is). The fifth and final member of the usual cabal, keeper of conscience and stone, is Dunstan.