I would like to know why the article "an" is used in the title:
An End, Once and for All
(a song title off a video game soundtrack)
However, The Beatles have a song called The End.
And is there a rule or it depends on the creators?
These two phrases mean two different things.
"An End, Once and For All" implies that this is an open-ended ending.
If this is from the Mass Effect 3 soundtrack as a simple Google search shows, it's probably pretty obvious to you that there were multiple endings and plot holes, ergo, the open-ended-ness of the song title.
"The End" implies nothing and lets us know quite firmly that this is the end.
It was a song that not marked the end of the medley, but also the last song recorded by the four Beatles collectively, and thus, appropriately enough, the end.
So it's important to note that context means (almost) everything, not only grammar.