svengali
One who manipulates or controls another as by some mesmeric or sinister influence; especially a coach, mentor or industry mogul.
She also offers a fascinating glimpse into a more innocent age before svengalis began colluding with marketing men and TV execs to promote talentless wannabes to overnight success.
Where it comes from
Named for the hypnotist character Svengali in George du Maurier's novel Trilby. More at citations.
Adapted from the Wiktionary entry svengali (CC BY-SA 4.0).