Demons is a Star Trek: The Original Series

Star Trek: The Original Series

Star Trek is an American science fiction television series created by Gene Roddenberry, produced by Desilu Productions . Star Trek was telecast on NBC from September 8, 1966, through June 3, 1969...

novel written by J.M. Dillard.

Plot

A strange device found by a scientific expedition is taken to the planet Vulcan. It begins taking people over one by one, replacing them with malevolent power-hungry entities. The crew of the Enterprise, those not yet replaced, must contain this threat to Vulcan and defeat it.

This story is continued in the Star Trek: The Next Generation

Star Trek: The Next Generation

Star Trek: The Next Generation is an American science fiction television series created by Gene Roddenberry as part of the Star Trek franchise. Roddenberry, Rick Berman, and Michael Piller served as executive producers at different times throughout the production...

novel Possession, also by J.M. Dillard, where it is revealed the device is one of many.