Kos'Karii encircling a Klingon

In Klingon mythology, the Kos'Karii were pale, serpent-like creatures which inhabited the blood-red waters crossed by the Barge of the Dead on its way to Gre'thor. They attempted to lure dishonored souls to them with voices of friends and loved ones, and then drag them into the water.

In 2376, while experiencing a ride on this barge, B'Elanna Torres was told by Hij'qa that the voices she heard were not her crewmates on USS Voyager, but the Kos'Karii. Later, Miral mistook her own daughter for one. (VOY: "Barge of the Dead")

The Star Trek Encyclopedia (4th ed., vol. 1, p. 439) described the Kos'Karii as a type of eel.