A New Hope opened with an thrilling assault on a rebel ship and as the Storm troopers gained access, Darth Vader famously stepped in, his black figure large and menacing.
Since that moment fans have wondered who this Vader guy was and how he came to be.
The original Trilogy films added small iterations to the back story. ANH told us he was a pupil of Ob Wan Kenobi, seduced by the dark side. Empire revealed Vader was Luke's father and Jedi confirmed he was indeed the chosen one but the full back story of who Vader really was and how he had ended up as the most feared person on the galaxy was never explored.
Enter the prequel trilogies:
In a weird kind of way you could think of Darth Vader as having a similar plight as King Kong for it were beauty that bring down the beast. What do we mean? Vader was once Anakin Skywalker, a powerful Jedi Knight who fell in love with Padmé and it was his love for her that was ultimately his undoing.
Anakin had a vision that plagued him of Padmé dying and he wished to prevent that - when this was made known to Palpatine, the then undercover Sith Lord seized an opportunity to use that need to finally turn Anakin against the Jedi and so succeed in his goal of gaining ultimate power.
While his motives for his actions in Revenge of the Sith were initially pure, his fear and anger lead to the Dark Side (as Yoda predicted).
Following his final encounter with Kenobi on Mustapha were he was left for dead and having fatally wounded Padmé (or possibly simply caused her to lose her will to live) - Vader had no choice but to continue the path set before him by his seduction by the Sith Lord.
He then set about furthering the Emperor's rule by seeking out and exterminating the remaining Jedi.
Vader's story came to an end in Return of the Jedi where he killed the Emperor out of love for his son Luke thus fulfilling the so called prophecy that he would bring balance to the Force.