Macdonwald is a rebel leader who fights against King Duncan at the very start of Macbeth. He is a Scottish traitor who joins forces with invading Norwegian troops in an attempt to overthrow the king.
Macdonwald never appears as a character onstage. Instead, he’s described in Act 1, Scene 2 through a battle report. A wounded captain explains that Macdonwald’s rebellion is fierce and dangerous, supported by foreign forces. Macbeth proves his loyalty and bravery by confronting him directly in battle and killing him in a brutal, memorable way: he “unseamed him from the nave to th’ chops,” essentially cutting him open from stomach to jaw.
This moment matters less for who Macdonwald is as a person and more for what his defeat establishes. Macbeth is introduced as a heroic defender of Scotland, someone who destroys a traitor threatening the natural and political order. His characterization here is ironic, because the man who begins the play by killing a rebel against the king will soon become a far worse traitor himself by murdering Duncan.
Macdonwald’s brief role sets up the idea that loyalty can easily turn into betrayal, and that the violence used to defend a kingdom can later be turned against it.