What is the name of the villain in 'The Adventure of the Empty House'?

The villain in “The Adventure of the Empty House” is Colonel Sebastian Moran.

Moran is introduced as a former army officer, expert marksman, and criminal associate of Professor Moriarty. After Holmes returns from his presumed death at Reichenbach Falls, Moran becomes the principal threat in this story. He murders the Honourable Ronald Adair with a specially adapted air gun after Adair discovers that Moran has been cheating at cards.

Holmes and Dr. Watson use a lifelike wax bust of Holmes as bait. Moran fires at the dummy from an abandoned house overlooking Baker Street, believing he has finally killed his old enemy. Inspector Lestrade then arrests him. Holmes describes Moran as the second-most dangerous man in London, with Moriarty ranked above him.

Moran is sometimes confused with Moriarty because both belong to the same criminal network, but they are separate characters. Moriarty is the mastermind; Moran is the skilled assassin and hunter. The story first appeared in 1903 and marked Holmes’s return after the “Great Hiatus.”