Star Trek: 10 Alien Encounters Before First Contact
5. The Devidians - Time's Arrow
The Devidians were time-traveling aliens who fed from the neural energy of their victims, a process which was ultimately fatal. These were not a species that were particularly interested in understanding those that they devoured, and merely sought to find more nourishment.
To that end, they found a way to travel back to San Francisco in the 19th Century. There, they masqueraded as humans, picking people off one by one, with a recent cholera epidemic taking the blame for their actions.
In a good old predestination paradox, the discovery of Lt. Commander Data's head prompted the beginning of the end of the Devidians. It was excavated in the 24th Century, thus leading the crew of the Enterprise-D to Devidia II, and then back to Earth itself. With a little help from Guinan and one Samuel Clemens, the Devidians were stopped in their efforts - with their base of operations destroyed via a volley of torpedos.
History repeated itself, with Data's headless body recovered from the away mission, and the now 400-year old head slapped back atop. All in a day's work when you're an android, really.