As Future Trunks attempted to make his way out of the dystopian future he lives in, the mysterious antagonist who has been pursuing him, and killed Future Bulma, finally reveals himself, and it appears to be none other than a future version of Goku! Of course, fans of the Dragon Ball Super dub series know that this villain's name is "Goku Black," and there's actually a pretty complicated backstory to the character. For those fans who are seeing the "Future Trunks Saga" for the first time, here's an answer to the question: Just who is Goku Black?
WARNING: Mild Spoilers Follow!
A Tale of Two Timelines
"Goku Black" is actually "Zamasu," a former North Kai / Supreme Kai apprentice to the Supreme Kai of Universe 10, Gowasu. There are actually two versions of the character that are relevant to the "Future Trunks Saga", based on two distinct timelines. Here's the one we've already met in the opening chapter of the Dragon Ball Super Dub:
Meet "Goku Black"
Goku Black is the present day version of Zamasu from the "Unaltered Timeline" where the two big time travel events of Dragon Ball Z's "Cell Saga" (Trunks returning to the past from Timeline 2, and Cell from Timeline 3 becoming "Perfect Cell") never occurred.
In this "Unaltered Timeline," the present day Zamasu killed his master Gowasu, in order to launch his "Zero Mortals Plan," as he believes mortals are destructive by nature - especially Saiyans like Goku, who can achieve power to rival the gods. After meeting Goku, and learning of the Super Dragon Balls, present day Zamasu used the balls to swap bodies with Goku, killing his old body with Goku trapped inside it. Taking the name "Goku Black," Zamasu uses Gowasu's Time Ring to jump to Timeline #2 where Future Trunks lives, thereby setting off the chain of time-altering events known as "The Future Trunks Saga" in Dragon Ball Super.
As stated, Goku Black is just one of the versions of Zamasu that is relevant to the "Future Trunks Saga". There's a whole lot more to the character's backstory - which we'll reveal when the Dragon Ball Super Dub gets a little further into the "Future Turnks Saga"!
Dragon Ball Super Dub airs Saturday nights @ 10:30/9:30c on Toonami (Cartoon Network).