Redjac (voiced by John Fiedler) was a malevolent, non-corporeal entity that appeared in the Star Trek episode "Wolf in the Fold". It had no true appearance as it possessed other life-forms which it would then use to kill and incite terror in other beings, feeding on the emotion of fear. Redjac had existed for countless centuries and was confirmed to be responsible for the legend of Jack the Ripper on Earth.
After Redjac committed a murder on the pleasure planet Argelius II in 2267 as "Hengist", Lieutenant Commander Montgomery Scott from the Federation starship Enterprise was accused of the crime. However, Hengist's true identity was later revealed. In the process, Redjac briefly possessed Prefect Jaris, who was quickly liberated by a nerve pinch from Spock. Subsequently, the Redjac entity returned to Hengist's body and was incapacitated. Hengist was then beamed, on a dispersal pattern, into deep space by Spock, where it was hoped that Redjac would die from lack of nourishment.