Mr. Hengist, a native of Rigel IV, was the chief administrator of the capitol city of Argelius II. When a series of murders took place, he believed that Montgomery Scott was the murderer.
In reality, Hengist was possessed by a non-corporeal lifeform named Redjac, known as Jack the Ripper on Earth. It fed on the fear of its victims, and it was responsible for the murders. When discovered, Redjac killed Hengist, and attempted to enter the Enterprise. When this was unsuccessful, it re-entered Hengist, re-animating the body. Hengist's body, with the lifeform in it, was beamed out into space in a wide dispersal pattern, killing the lifeform. (TOS: "Wolf in the Fold")
Mr. Hengist was played by actor John Fiedler.