It is in Act III, Scene II that Macbeth invokes the night through the words, "Come, seeling night." To put this into context, Macbeth is about to send his henchmen to kill Banquo and his son, Fleance, and he begs the night to come so that its darkness might conceal these murders from others ("blindfold the kindhearted day").
As Macbeth continues, he urges the night to use its "bloody and invisible" hand to commit these murders and so bring about an end to Macbeth's "fear." Remember that Macbeth is afraid of the prophecy in which it was foretold that Banquo's sons would be the kings of Scotland. As such, Macbeth wants to protect his crown and is prepared to go to any lengths to ensure that Banquo's sons do not take it from him.
In the next lines, Macbeth likens himself to a predator who awakens in the night to hunt his prey, represented here by Banquo and Fleance. This is significant because he is suggesting that these murders are legitimate--that they are an inevitable part of the natural order and that they cannot be avoided.