What is the name of the villain in 'The Adventure of the Illustrious Client'?

The villain in “The Adventure of the Illustrious Client” is Baron Adelbert Gruner.

Gruner is an Austrian baron portrayed as a wealthy, cultured and exceptionally dangerous criminal. He is suspected of murdering his first wife and is described as a ruthless philanderer who keeps a written record of his romantic conquests. The case begins when an unnamed illustrious client asks Sir James Damery to prevent Gruner from marrying Violet de Merville.

Holmes tries to persuade Violet that her fiancé is a sadistic murderer, but Gruner's charm and manipulation initially defeat that effort. Holmes therefore seeks stronger evidence. With help from the informer Shinwell Johnson and Gruner's former mistress Kitty Winter, he obtains the incriminating book from Gruner's study. When Violet sees it, she finally accepts the truth and breaks off the engagement.

Professor Moriarty and Colonel Moran are famous Holmes villains, but neither is the antagonist of this story. Gruner is defeated not by a conventional arrest or courtroom revelation, but by the evidence of his own catalogue of victims. The story was published in 1924.