An inhabited world in the Rigel system, home of the administrator Hengist and scene of unsolved murders of women revealed to be the work of the entity Redjac.
Rigel IV is one of several inhabited planets of the Rigel system. It features in the background of the original series episode Wolf in the Fold. The murder weapon used on Argelius II is a knife made by the hill people of the Argus River region of Rigel IV, and the Argelian official Hengist comes from that planet.
The Enterprise computer links the Argelian killings to a string of unsolved murders of women on Rigel IV, attributed to a killer called Beratis. All prove to be the work of Redjac, the fear-eating entity once known as Jack the Ripper, which had taken Hengist's form.
The planet is also mentioned in passing in later series, for example as the venue of a hydroponics conference attended by Keiko O'Brien in The Wire.