Thane of Fife
Fife
Lady Macbeth
Macbeth is first 'Thane of Glamis' then he is bestowed the title 'Thane of Cawdor' then king of Scotland
He must watch out for Macduff
Duncan appoints him to that title, after it was stripped from the previous Thane. The former Thane of Cawdor was stripped of all titles and executed after joining a Norwegian invading force which was defeated at Fife. Macbeth at the time was at Forres, nowhere near Fife.
=Macduff The Thane of Fife=
Perhaps you are thinking of Lady Macbeth's line during her sleepwalking scene: "The thane of Fife had a wife--where is she now?"
Macduff, the Thane of Fife, kills Macbeth.
"Beware Macduff, beware the Thane of Fife."
When Macbeth refers to Macduff he is talking about a man who holds the title Thane of Fife.
Malcolm (Duncan's son) did not kill Macbeth, it was Macduff (Thane of Fife).