controlling influence
131tail wagging the dog — This expression refers to a situation where there is a reversal of roles, with the small or minor element having a controlling influence on the most important element. If you let your children decide on everything, it will be a case of the …
132Svengali — [svɛn gα:li] noun a person who exercises a controlling influence on another, especially for a sinister purpose. Origin Svengali, a musician in George du Maurier s novel Trilby (1894) who controls Trilby s stage singing hypnotically …