Old Siward is first mentioned in Shakespeare's Macbeth in act 3, scene 6. Lennox is speaking with a lord about transferring his allegiance from "the tyrant" Macbeth to Malcolm, the the son of the murdered King Duncan and the rightful heir to the throne.The lord tells Lennox that Macduff has gone to England to enlist the English king's aid against Macbeth, along with the aid of "Northumberland and warlike Siward" (2.6.32).
Malcolm later tells Macduff that "Old Siward, with ten thousand warlike men" has joined their cause (4.3.149). Not only is Old Siward "warlike," but his ten thousand soldiers are "warlike" as well, which doesn't bode well for Macbeth.
In act 5, scene 2, Menteith mentions that Siward is Malcolm's uncle. Lennox says that Siward's son is with him as part of his army and that this will be Young Siward's first battle.
In act 5, scene 4, Young Siward is with his father when Malcolm's army approaches Birnam Wood, and Malcolm gives an order to his soldiers that will strike fear in Macbeth's heart:
MALCOLM. Let every soldier hew him down a bough,
And bear't before him: thereby shall we shadow
The numbers of our host, and make discovery
Err in report of us. (5.4.6-9)
In act 5, scene 6, Malcolm's army, camouflaged to resemble Birnam Wood, reaches Dunsinane castle. Malcolm orders his army to throw down the branches that they had been using to camouflage themselves. He then gives the honor of leading the first battle against Macbeth to Old Siward, with Young Siward at his side.
MALCOLM. Now near enough; your leavy screens throw down,
And show like those you are. You, worthy uncle,
Shall, with my cousin, your right noble son,
Lead our first battle. (5.6.1-4)
Malcolm tells Old Siward that he and Macduff will follow with the reserve army. Old Siward, an old soldier and commander of many battles, jokes that if his army does their job well enough, there won't be anything left for Malcolm and Macduff to do.
OLD SIWARD. Fare you well.
Do we but find the tyrant's power tonight,
Let us be beaten, if we cannot fight. (5.6.5-7)
Old Siward doesn't appear in act 5, scene 7, but in this scene, his young son meets Macbeth in battle. Young Siward doesn't recognize Macbeth, as he's never seen him before.
YOUNG SIWARD. What is thy name?
MACBETH. Thou'lt be afraid to hear it.
YOUNG SIWARD. No, though thou call'st thyself a hotter name
Than any is in hell.
MACBETH. My name's Macbeth.
YOUNG SIWARD.The devil himself could not pronounce a title
More hateful to mine ear. (5.7.5-11)
Young Siward fights Macbeth, but Macbeth, supremely confident because he's been assured by the Witches that "none of woman born / Shall harm Macbeth" (5.1.88-89), easily overpowers and kills Young Siward.
In act 5, scene 8, Macbeth's army has been defeated, and Macduff, who is "none of woman born," has killed Macbeth.
Old Siward learns from Ross that Young Siward has been killed in battle:
ROSS. Your son, my lord, has paid a soldier's debt:
He only lived but till he was a man;
The which no sooner had his prowess confirm'd
In the unshrinking station where he fought,
But like a man he died. (5.8.44-48)
Old Siward shows the true character of the old soldier that he is—stoic, proud, and honorable. Old Siward is grieved at his son's death, but he's proud to know that his son died a noble death.
OLD SIWARD. Why then, God's soldier be he! ...
They say he parted well and paid his score:
And so God be with him! (18.104.22.168-61)