Kos'Karii

The Kor'karii are a mythical beast within Klingon beliefs. The creatures are said to dwell within the watery wastes of the underworld, Gre'thor. They were pale, serpentine beings who would swim near the Barge of the Dead where they would use their siren-like to disguise themselves as the voices of friends or loved ones in order to draw the dishonored warriors into the water where they would be lost forever.

(VOY episode: Barge of the Dead)