Cargûl are found wandering in the instance Carn Dûm, outside the gate of Tármunn Súrsa. Level 75 Cargûl can also be found in south-central Trollshaws, although they are mainly meant to be avoided rather than fought.
Cargûl (Meaning "Red Wraith" in Black Speech) are lesser wraiths enslaved by the Nazgûl. Most are likely victims of the Nazgûl's Morgul-blades, a terrifying supernatural weapon that saps a person's life. Those wounded by a Morgul-blade will find themselves fading from the mortal world and drawn into the wraith-world, a spiritual plane just beyond our perception of reality. Once the transition is complete, the person is transformed into a wraith and, like the Nazgûl themselves, can only be seen by mortal eyes when enshrouded in dark robes. The Cargûl are hapless souls that have fallen victim to this calamity.
Though J.R.R. Tolkien does not use the term Cargûl, he does establish the transformation that a victim of a Morgul-blade undergoes. Frodo Baggins nearly succumbs to this condition himself when he is stabbed by the Witch-king at Weathertop.