What poetic device is used in Act 1, Scene 1 of Macbeth and what effect does it have?
There are several poetic devices used in this short scene. You can read the scene with annotations and a modern translation on eNotes here.
The first is pathetic fallacy. Pathetic fallacy is a literary device where the environment, such as the weather, reflects the feelings of the characters, adding to the atmosphere or mood. In Act I scene I, the stage directions call for "thunder and lightning," and the witches mention terrible weather patterns including thunder, lightning, rain, fog, and filthy air. The sound of thunder, flash of lightning, and references to bad weather in the first act and scene of the play establish a scary, foreboding atmosphere.
Another literary device that is used in this scene is foreshadowing. Foreshadowing occurs when a character or event hints at an event that will happen later in the work. In this case, the Three Witches foreshadow the battle: "When the battle's lost and won," and then they foreshadow their meeting with Macbeth: "There to meet with Macbeth." This lets the audience know that the Three Witches have the power of precognition and that they will influence Macbeth in the course of the story.
Thirdly, the device of paradox is used in Act I scene I, and continuously throughout the play. Paradox is a literary device where opposites are contrasted. The witches agree to meet after "the battle's lost and won." Here the paradoxes "lost" and "won" are contrasted to show that the witches don't care who wins or loses the battle. This indicates that they don't care about politics; they just want to stir up trouble. Then the Three Witches recite the famous line, "Fair is foul, and foul is fair," which paradoxes all that is "fair" (good, right) against all that is "foul" (evil, bad.) This shows the audience that the play is going to be filled with good against evil, right against wrong. Indeed, Macbeth and Lady Macbeth both experience inner conflict: good vs. evil.