A colossal single-celled organism thousands of kilometers long that drained energy from everything around it, destroying a star system and a starship crewed by Vulcans.
The space amoeba was a single living cell of staggering size, with a nucleus, protoplasm and an outer membrane surrounded by a zone of energy drain. It fed on energy and was close to dividing when the Enterprise met it.
In The Immunity Syndrome, it had already killed the billions of people in the Gamma 7A system and destroyed the USS Intrepid with its Vulcan crew, deaths Spock felt from afar. Spock flew a shuttle into it to study it, and the Enterprise followed inside. The crew destroyed the organism with an antimatter charge at its nucleus.