A bodiless entity that fed on terror, killing women on many worlds over the centuries, including on Earth as Jack the Ripper.
Redjac was a being of pure energy that sustained itself on fear, especially the terror of women. It took human hosts and moved from world to world, leaving murders behind it. Spock traced its history through the ship's records to Jack the Ripper on Earth and similar killings on other planets.
In Wolf in the Fold, women were killed on Argelius II and Montgomery Scott was accused. The killer was hiding inside the administrator Hengist. When it fled into the Enterprise computer, Spock ordered the computer to calculate pi to the last digit, driving it out. It was beamed into space at maximum dispersal.