The crack of doom
[A show of eight Kings, the eighth with a glass in his hand, and Banquo last]
Macbeth:Macbeth Act 4, scene 1, 112–117
Thou art too like the spirit of Banquo; down!
Thy crown does sear mine eyeballs. And thy hair,
Thou other gold-bound brow, is like the first.
A third is like the former. Filthy hags,
Why do you show me this?—A fourth? Start, eyes!
What, will the line stretch out to th' crack of doom?
In Shakespeare's time, "doom" still had the primary sense "judgment"; and here he uses it as shorthand for "doomsday" or Judgment Day, not as a metaphor for personal failure. The "crack" itself could be one of many things: the peal of an archangel's trumpet; one of Jehovah's thunderbolts, calling all souls to order; or even the verge of the abyss separating the temporal world from the final kingdom. This last meaning would rely on a relatively new sense of "crack"—as "break" or "fissure," a sense developed only in the sixteenth century. In any case, the "crack of doom" is, as far as Macbeth is concerned, a long ways off.
When, way back in Act 1, three witches prophesied that Macbeth would be king, they also told his friend Banquo that he would beget a dynasty. This prediction has haunted Macbeth, who has indeed become king in accordance with the prophecy; he thinks the crown worthless if he cannot establish a dynasty. After having Banquo murdered, and unsuccessfully attempting to have Banquo's son murdered, Macbeth revisits the witches. When he asks if Banquo's issue will ever really reign in Scotland, Macbeth is shown a pageant of eight kings. These kings represent the Stuart line, legendarily descended from Banquo. At the time of Macbeth's writing, a Stuart-James VI of Scotland—had recently ascended the throne of England as James I. Macbeth's despair over the number of kings—and their family resemblance to Banquo—is therefore flattery of James, whose line, Shakespeare suggests, will rule Scotland and England forever.