This is someone who dominates another person and manipulates them to achieve gain from their talents.
An example would be a talent manager who latches onto a young star and forcefully guides their career to make themselves rich, ignoring what they actually want to do with their lives.
It comes from the name of a character in a 1894 novel that does exactly this for a young singer.
Why I Looked It Up
I knew the gist of the word, but wanted to know where it came from.
Then in Conspiracy of Fools, Louis Borget is described as:
Enron’s earnings svengali
This is an interesting usage. It implies that Borget dominated and forcefully used Enron to earn money for himself – which is actually pretty accurate to history.