Final answer:
The speaker is the ghost of King Hamlet from Shakespeare's play, revealing his murder by his brother Claudius and the subsequent usurpation of his throne and marriage to his queen.
Step-by-step explanation:
The speaker of the quote "Thus was I, sleeping, by a brother's hand / Of life, of crown, of queen, at once dispatched..." is the ghost of King Hamlet from William Shakespeare's play Hamlet. In this line, the ghost reveals to his son, Prince Hamlet, the truth about his murder. He tells Hamlet that while he was sleeping in his orchard, he was poisoned by his brother Claudius, who not only took his life but also his throne and his queen, Gertrude.
Shakespeare's play Hamlet deals with themes of revenge, betrayal, and the complexity of the human condition, represented here by the ghost's revelation of his murder and the consequences it invokes.