A svengali is a person who controls another’s mind or has the ability to control others, usually with sinister intent. In the 1891 gothic-horror novel Trilby, by George du Maurier, Svengali is a hypnotist who transforms the title character into a famous singer. The book was popular in its time, so the reference would have been familiar to most readers of literature in the early 20th century, when svengali entered the language.
Name-derived words that stay in the language usually lose the initial capital sooner or later, depending on how common they are. Svengali is still usually capitalized, though the uncapitalized form is now fairly common.
This early 20th-century serial killer gives the lie to the sexist idea that women can only murder properly under the influence of svengali men. [Guardian]
[I]t was Brian Epstein, who emerged as a figure of fascination in his own right, anointed by the press as the new Svengali of pop. [Can't Buy Me Love, Jonathan Gould]
To this day, he does not seem to realize that Dick Cheney was both vice president and White House svengali. [Washington Post]
Like its predecessors, One Direction is the creation of a Svengali known for his merciless pursuit of chart success at all costs. [AV Club]
In the 1960s came the academic specialists—McGeorge Bundy, Walt Rostow, Henry Kissinger, Zbigniew Brzezinski—svengalis who would tutor the president in the occult science of foreign aflfairs. [The Korean War, Bruce Cumings]
The former are the Svengalis, trying to coax and even trick the performance they want out of the latter. [Independent]