Siéhnams appear to the untrained eye like ordinary pampas deer, with similar coloration and shape if somewhat larger than their ordinary cousins. However, they are savage killers, roaming not only their high plateaus and rolling prairies but also invading settled areas to hunt those wandering about by night and slaughtering those they discovered in their sleep. Some legends say the first siéhnams were living manifestations of nature's vengeance, their eyes turning red and their mood feral as they sought reprisal for the killing of their mates and fawns. Most ordinary deer avoid them, but some who have suffered great loss or who have survived the destruction of their homes and herds find themselves drawn to the roaming siéhnam, mating with and joining them to swell their numbers.

A siéhnam's antlers are sharper than knives and their teeth are adapted with oversized canines for ripping and tearing flesh. While they delight in all forms of killing, humanoids are their favored prey, and they love nothing better than sinking their fangs into the throat of a sleeping victim and exulting in their bloody demise.