Demons (Star Trek: The Original Series, #30)

Long before the Federation, powerful force invaded our galaxy and almost destroyed it... a force that began with possession and madness, and ended in murder!
A Starfleet research expedition to the farthest reaches of the galaxy has unearthed that force once again... and brought its silent evil back to the planet Vulcan. Now Spock must defeat the demons that threaten his friends and family,or the Enterprise™ will become the instrument of the galaxy's destruction!