Final answer:
In Shakespeare's 'Macbeth', the former Thane of Cawdor, who betrays King Duncan, is not named in the play; however, his treachery leads to his execution and the title is given to Macbeth.
Step-by-step explanation:
The former Thane of Cawdor who betrays King Duncan at the beginning of William Shakespeare's play 'Macbeth' is never actually named in the text. However, it is clear that this unnamed Thane of Cawdor was found to be a traitor to King Duncan of Scotland. Upon his betrayal being discovered, King Duncan orders his execution and confers the title of Thane of Cawdor upon Macbeth for his loyalty and bravery in battle. This act of betrayal is significant, as it not only sets the stage for Macbeth's rise to power but also reflects the theme of treachery that permeates the play.