The damned spot that Lady Macbeth refers to is King Duncan's blood, in William Shakespeare's play "Macbeth." The play is a tragedy that is also known by its complete title, "The Tragedy of Macbeth." It is the shortest tragedy written by Shakespeare.
The words that refer to the damned spot in "Macbeth" are, "Out, damn'd spot! out, I say!" These lines are delivered by Lady Macbeth, who starts to hallucinate after she convinces her husband Macbeth to murder King Duncan, the King of Scotland. Macbeth and Lady Macbeth, who are now King and Queen of Scotland, are stricken with grief and torment after King Duncan's death, which ultimately results in their death as well.