Whisper hounds are enchantments used to protect areas, such as dungeons and treasure rooms. One hound is in the Fablehaven dungeon. Once a whisper hound has your scent, escape is futile.
While captured, Torina Barker used a whisper hound to prevent Kendra Sorenson from escaping. Kendra described the feeling of a whisper hound as 'a rush of cold air'.