Ishmael (Star Trek: The Original Series, #23)

The Enterprise is on a peaceful mission at Starbase 12, and Spock is visiting aboard a Klingon vessel, when a mysterious phenomenon causes the Klingon ship to vanish. Spock's last message from the ship is cryptic, but frightening. It suggests that the Klingons are traveling into the past, hoping to kill one man who was decisive in the course of history, and thereby change it, destroying the Federation before it was born.