In the New Oxford Review, a board-certified psychiatrist and professor, Richard Gallagher documents a real Case of Demonic Possession.
Even those who doubt such a phenomenon exists may find the following example rather persuasive. For clergy, or indeed anyone involved in the spiritual or psychological care of others, it is equally critical, however, to recognize the many and infinitely more common "counterfeits" (i.e., false assignations) of demonic influence or attack as well.
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We made a number of phone calls to arrange gathering together to help Julia. Julia herself was not in on these phone discussions; she was far from the area at the time. Astonishingly, Julia's "other" voice -- again sometimes deep, sometimes high pitched -- would actually interrupt the telephone conversations and somehow come in over the phone line! The voice(s) would espouse the same messages: "Leave her alone," "Leave, you idiots," "Get away from her," "She's ours." Julia, again, said later that she was unaware of any such conversation. And yet this speech was heard distinctly by several of the team on a number of occasions.