Instrument of darkness- Explain.

This phrase is addressed to the witches from the part of Banquo. Banquo is very much realistic. Hus subsequent comment on the witches a little later is that the creatures whom they met were the agents of the devil, who mislead human being by telling them certain trivial truth and then betraying them in matters of vital importance. He thus expresses this view:
          "And often time, to win us to our harm. The instument of darkness tell us truths, win us with honest trifles, to betray us
In deepest consequences."

         Banquo knows the reality. So he warns Macbeth against danger of trusting the witches absoluyely. The word 'darkness' has a significant meaning. Apparently, it suggests that they are like the stormy, darkening weather. But implicitly it suggests that they transform the life of one from fair to foul means they help to makes one's life dangerous.