It looks like Marvel Comics may have just revealed where the title for Avengers: Endgame came from. Marvel has announced that they are launching their "True Believers" series, which are reprints from some classic comics that will be released to coincide with the highly anticipated movie in April. Two of the upcoming comic books have to do with the Black Knight, which has led many to believe that the character, or at the very least his Ebony Blade, will be introduced into the Marvel Cinematic Universe with Endgame.
As for the title Avengers: Endgame, that name has been floating around for quite some time, way before it was unveiled as the official title back in December. For one, Doctor Strange mutters the title right before the Decimation in Infinity War, which gave many fans all of the clues that they needed. For Hardcore fans, they already knew that Avengers #71 from 1969 was named Endgame! It is one of the two releases for the "True Believers" series that will be released in the spring.
The Black Knight is involved heavily in Endgame! and it is the second part of a series where the Avengers failed the first time as they went up against Squadron Sinister. However, things weren't exactly as they seemed. The Avengers were used as a pawn by Kang who was battling the Grandmaster over the Earth. In the first part of the series, Black Knight interfered with Hawkeye's battle against the Whizzer, resulting in a loss for the Avengers. Infinity War may have been like the first part of this battle and Avengers: Endgame is like the second in a loose way.
In Endgame! the heroes are given a second chance and had to go back in time to defeat Captain America, Namor, and the original Human Torch. However, Kang came out and betrayed the Avengers by killing all of them. The Black Knight was able to defeat Kang and then the Grandmaster revived all of the Avengers. Obviously, some of these characters were not in Infinity War and more than likely will not be in Avengers: Endgame either. But, it does lay the groundwork for the title and the basic premise for the upcoming movie.
Even if the Black Knight doesn't show up in Avengers: Endgame, Carol Danvers could be taking that spot in a change from the source material. Marvel Studios has not been taking stories directly from one or a series of comics. Instead they have been cherry picking and making new stories that are based on the original comics. With that being said, the 1969 Endgame comic is more than likely where Endgame got its title and some of the storyline. The studio isn't about to put out a huge spoiler right before the release of the movie. But, there are two more comics in the "True Believers" series that are still marked as confidential at this time, so they might provide more information than the studio is willing to release. You can check out the "True Believers" series over at Marvel.