Count Negretto Sylvius is the main antagonist of the Sherlock Holmes story "The Adventure of the Mazarin Stone"
A Jewel thief, and the mastermind of the robbery of the Crown diamond, a 100,000 pound stone. Holmes was hired to investigate the case, and having tracked down the Count and his muscle, a thug called Sam Merton. He formulates a plan to catch them, by inviting them to his home to take the Stone from them to save them from going to prison.
Arriving Holmes lays down his terms, and pretends to go into his room to play his Violin while they discuss it. The Count pretends to go along with this, but secretly reveals to Sam, he will double cross Holmes and claim the Jewel is hidden somewhere far away, while they have it smuggled to Holland early.
When pressed by Sam, he reveals the Jewel is on his person as he couldn't risk leaving it alone, due to the fact that if he could take it out of the vault, then someone else could take it out of where he left it. He shows it to Sam, Holmes then springs his trap, revealing a secret way from his bedroom back behind the curtain, and using a Gramophone to fool the criminals into thinking he was still in there. Having called the police before, they are caught red handed, and arrested.