Possession (Star Trek: The Next Generation, No. 40)

From the back cover: Eighty years ago, bodiless entities brought a plague of violence and bloodshed to the planet Vulcan. The nightmare ended only when the entities were trapped inside special containers.

Now, on the eve of a galaxy-scale scientific exposition, the containers have been opened, freeing the malevolent entities to possess the minds and bodies of all they encounter, including the crew of the Starship Enterprise. Friends turn into foes, and no one can be trusted, as Captain Picard faces a deadly and insidious threat. Unless the entities can be stopped once more, they will spread their madness throughout the entire Federation.