Macdonwald is not a character who appears in the play. He is the leader of the rebel forces fighting against the King of Scotland. He is mentioned in Act I, scene 2, when Macbeth is praised for defeating Macdonwald in battle. Macbeth is given two new titles as a result of this victory. You might say this victory is actually the beginning of Macbeth's downfall.

Although the characters in this play are historical figures, the play itself does not follow historic fact. According to Raphael Holinshed, on whose chronicles Shakespeare based the play, knowing that he would be executed, Macdonwald retreated to his home, killed his wife and children, and then committed suicide. Macbeth found the bodies and decapitated the dead rebel.

What happens to Macdonwald in Macbeth?

It should also be noted in this regard that Macdonwald is not presented in the play physically, but the audience come to know about him in Act I, Sc. ii when the “bleeding Captain” (referred to by Duncan as “the bloody man”) informs Duncan how Macbeth has defeated Macdonwaald and has “fix'd his head upon [their] battlements: "

Doubtful it stood,
As two spent swimmers that do cling together (10)
And choke their art. The merciless Macdonwald—
Worthy to be a rebel, for to that
The multiplying villainies of nature
Do swarm upon him—from the western isles
Of kerns and gallowglasses is supplied; (15)
And fortune, on his damned quarrel smiling,
Show'd like a rebel's whore, but all's too weak;
For brave Macbeth—well he deserves that name—
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution, (20)
Like valor's minion carved out his passage
Till he faced the slave,
Which ne'er shook hands, nor bade farewell to him,
Till he unseam'd him from the nave to the chaps,
And fix'd his head upon our battlements. (25)

Finally it also needs to be mentioned that Shakespeare does not provide further details about Macdonwald in the text.

What happens to Macdonwald in Macbeth?

Macbeth defeated Macdonwald in battle at the beginning of the play.  In Act I, scene ii, a sergeant reports to Duncan that Macbeth killed Macdonwald in the battle.  The sergeant describes the battle between the two men and uses the simile of "two spent swimmers" to describe the fight.  At the end he says that Macbeth "unseam'd him from the nave to the chaps, / And fix'd his head upon our battlements."  In other words he cut him open from his gut to his chin and then cut off his head and put it up for everyone to see.  This account showed how strong Macbeth was in battle and that he could be a ruthless fighter.