Redjac
Redjac (also known as Red Jack, Jack the Ripper, Beratis, and Kesla) was a non-humanoid lifeform that existed for centuries by journeying from planet to planet and feeding on the pain and fear he caused by committing mass murders. ()

Physiology 

As a being without form, which was likened to a cloud creature, its physical appearance unknown, but it was described by Mr. Scott after a brief encounter in the dark as, "cold… like a stinking draft out of a slaughterhouse, but it wasn't really there." 

Spock believed that female victims were chosen more often than male ones because "women are more easily and more deeply terrified, generating more sheer horror than the male of the species."

It was similar to a parasitic organism, and required the possession of a humanoid host in order to commit crimes. It also had the ability to possess objects, like the .  ()

History 

Crimes committed under various pseudonyms:

 On Earth and Mars:
 19th century: several women as Jack the Ripper
 1932: Seven women in Shanghai, China
 1974: Five women in the city of Kiev, USSR
 2105: Eight women in the Martian colonies
 Outside Sol system:
 2156: Ten women in Heliopolis on Alpha Eridani II 
 Interim: several women (as "Kesla") on Deneb II.
 2266: several women (as "Beratis") on Rigel IV
 2267: several women (in the form of Hengist) on Argelius II

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 was beamed, on a dispersal pattern, into deep space by Spock, where it was hoped that it would die from lack of nourishment. ()

Appendices

Apocrypha 
In James Blish's novelization of the original script, Redjac manifested itself on the viewscreen in the form of a Dantesque scene, with fire and brimstone, naked women riding on the backs of satyrs, horned toads, etc. Kirk declared that it was a vision of Hell, whereby the "filthy thing" was showing the Enterprise crew where it was born.

In a  story released by WildStorm Comics titled "Embrace the Wolf", it was revealed that Redjac survived by latching on to a nearby starship. After an absence of over a century, Redjac returned to confront the crew of the , where, having provoked a nuclear war on a Federation planet, he transferred himself into the holodeck and abducted several crewmembers, challenging Data in his Sherlock Holmes persona to save them, while Redjac manifested a body for himself in the form of a skeletal figure with glowing eyes. Data was able to save Counselor Troi from Redjac, and was also able to contact Captain Picard, Dr. Crusher, and Lt. Worf. Tricking Redjac into a confrontation with Worf, the crew was able to have Redjac focus all his energy on one spot, before trapping him in a photon torpedo. The torpedo was subsequently abandoned on a nearby barren moon.

Redjac also returned in two issues of the first Star Trek series from DC Comics, in a two-part comic with the titles "Wolf on the Prowl" and "Wolf at the Door". In this story, the entity possessed Enterprise crewperson Nancy Bryce and forced her to kill a number of ship personnel. Redjac had been revealed to have spent the twenty years between the TV episode and the comics on a primitive planet where it set itself up as their "God" and presented an image of Kirk in an opposite role as a "Devil" in the context to this "God". Redjac was defeated by following the Excelsior into a wormhole, which was then collapsed as the ship exited, trapping Redjac inside.

External links 
 
 
 

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Non-corporeals
Individuals
Nicknames