Grand Moff Tarkin was the Governor of the Imperial Outland Regions. He was appointed Grand Moff - the only Imperial officer to hold such a title - by Emperor Palpatine. He was so close to the Emperor that he was given some authority over even Darth Vader (as evidenced in a meeting of Imperial General Staff when he orders Vader to stop 'the grip' on General Motti). Tarkin's ambition to succeed Palpatine as emperor had led him to use extreme methods to quell the rapidly growing Rebel Alliance. His major project was to undertake the construction of the Death Star battle station (he is seen briefly in 'Revenge of the Sith' next to the Emperor & Vader, watching the construction of the big, steely orb that will become the Death Star). When the Death Star was completed, it was to be used as a base to rule the entire galaxy. With a superlaser capable of destroying an entire planet, it was considered by Tarkin & other Imperial General Staff to be invincible.
When the Death Star's plans are stolen, the Emperor assigns Darth Vader to recover them - and the recovery attempt results in the capture of Princess Leia but fails to recover the plans. Tarkin is obsessed with finding the Rebels' main base, and he has Vader resort to extreme measures - such as torture - to find out that location. Even though Vader has her tortured, she tells him nothing, and Tarkin suggests that threatening the Princess' home world - Alderaan - with the Death Star's destructive power. Tarkin's bluff works - to a point, as Princess Leia gives him the name of Dantooine. But the fiendish Tarkin orders Alderaan's destruction all the same, and then finds out that Dantooine was NOT the Rebels' main base. Tarkin gives the order to terminate her, but she is rescued by Luke Skywalker, Han Solo & Chewbacca before it can be carried out. When he finds out the Princess has escaped, Tarkin comes up with a plan; he will have a homing beacon placed on Han Solo's 'Millenium Falcon' in the hope that if the ship escapes, it will lead them to the secret Rebel base. With the tractor beam put out of action by Ben Kenobi, Tarkin's plan works, and the Death Star is led to the fourth moon of Yavin. However, the Rebels launch an offensive attack against the Death Star, consisting of dozens of X- and Y-Wing fighters. They are too fast for the Death Star's slow-moving turbolasers, designed for use against capital ships. Tarkin, however, believes his battle station is invincible, even when told by one of his officers that the attacking Rebels do present a danger. The Grand Moff stubbornly refuses to leave his post in the Death Star's main command center, and watches as the fourth moon comes up on the Death Star's targeting screen. Seconds after giving the order, 'You may fire when ready', Tarkin is dead - the Death Star is blown apart when Luke Skywalker's torpedoes find their mark and strike the reactor.