Right after King Duncan is murdered by Macbeth, Lennox reports some strange occurrences during that night. He proclaims that people have reported an earthquake and heard cries of sorrow, unusual screams of death, voices predicting catastrophes and the cries of the owl:
The night has been unruly: where we lay,
Our chimneys were blown down; and, as they say,
Lamentings heard i' the air; strange screams of death,
And prophesying with accents terrible
Of dire combustion and confused events
New hatch'd to the woeful time: the obscure bird
Clamour'd the livelong night: some say, the earth
Was feverous and did shake.
These occurrences are the external manifestations of the chaos created when Macbeth killed Duncan. Macbeth destroyed the natural order of things, and this is reflected in the nature. By killing Duncan, who was the lawful King of Scotland, Macbeth throws the whole country into disorder. Even nature protests against Macbeth's action and suggests that the world has been thrown into the hands of an inexorable villain.
Right after we hear of these unusual events, Macduff, horrified, states that the King has been murdered. Macbeth puts an end to the natural order of things in order to cling to his "black and deep desires," which ultimately lead to his imminent downfall.