In Act 1 Scene 5, Lady Macbeth says, "Come, you Spirits that tend on mortal thoughts, unsex me here, and fill me, from the crown to the toe, top-full of direst cruelty!" (I.v.40-43). She does not believe that Macbeth has enough cruelty in his heart to follow his ambition and do whatever he needs to do to become King. This scene relates to an earlier event in Scene 3 when Macbeth ponders the witches' prophecy. He has just been given the title Thane of Cawdor as the witches have said. Macbeth thinks that this has been an act of fate, so he reconciles to wait for fate to make him king: "If Chance will have me King, why, Chance may crown me, without my stir" (I.iii.143-144). Lady Macbeth is right about her husband, so she decides to become his sense of ambition to push him to act.
Since this scene is our first meeting of Lady Macbeth, I'm going to assume that you are seeking out the speech that begins: "The raven himself is hoarse," and that the "earlier thoughts" have to do with other characters earlier in the play.
Well, in this speech, Lady Macbeth is calling on the forces of darkness to make her strong and resolute, like a man, not a soft and motherly woman, so that whatever needs to take place (murder), so that Macbeth can become King, will. She concludes:
...Great Glamis! Worthy Cawdor!
Greater than both, by the all-hail hereafter!
These words and the incantation that calls upon unseen forces of darkness relate back to the witches in Act I, scene iii. They greet Macbeth with "All hail, thane of Glamis" and "All hail, thane of Cawdor" and finally "All hail, Macbeth, that shalt be king hereafter."
So, the forces of evil or darkness are suggested to be at work in Macbeth's rise to power. First in the witches' prophesies and then echoed by Lady Macbeth.