Re: "The Legend Ends"???
Well, it can also be taken as the trilogy is ending, being this is the last movie in this series. So this legend of Batman, is coming to an end.
It could also stand for him dying, or him just giving up the cowl.
As for the statement at hand, I think it's just a way to get hype, and is referring to it being the end of this series. I don't think they would blatantly spoil the movie with a tag line like that. That'd be too obvious.