Although deus ex machina didn't show up in Elliott Ackerman's novel, it probably does show up in the next mystery, recommended by a friend, that I read: The NYT best seller The Thursday Murder Club by Richard Osman.
In the last 75 pages, the protagonists travel outside of England to Nicosia to meet with a patriarch Demir Gundoz (interesting last name). His son Johnny is introduced late in the mystery, a form of deus ex machina. Most cosy mysteries have characters travel from the British countryside to London, in comparison. Break the rules! seems to reign supreme in mysteries.
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Osman ends the mystery with more than one demise.
