Musar על ערכין 30:21

Now when we come to assess the greatness of the sin of lashon hara, the damage one can do with his tongue, we find ourselves powerless to do so. For it permeates all of Torah, Prophets, Writings, and the apothegms of our sages of blessed memory. And, in general, one knows that he undermines thereby [the processes of] the upper worlds, [strengthening thereby the Sitra Achara (the "Adversary") and giving it power to incriminate the entire world.] And, below, he spoils the entire creation and himself, [for he thereby draws down the spirit of uncleanliness on all the fibers of his soul, as we find in the holy Zohar, Metzora.] And he harms others, as our sages of blessed memory have said (Arachin 15b): "The third tongue [i.e., the tongue of the talebearer, which is the third between a man and his neighbor, to reveal secrets to him] kills three: the speaker [of lashon hara], the accepter, and the object." And, in general, they said (Ibid.): "All who speak lashon hara magnify sin until the heavens, as it is written (Psalms 73:9): 'They set their mouths in the heavens when their tongues walk the earth.'" The plain explanation is that they thereby arouse the great Prosecutor against Israel, and bring death, sword, and slaughter to the world, as we find in the holy Zohar (see Chapter III).