We've seen plenty of fan trailers for imagined DC vs. Marvel films, but few are as seamless as this one for "Worlds Collide: Avengers vs. Justice League."
Using dialogue from X-Men: Age of Apocalypse and The Flash, and footage from a variety of sources, including Avengers: Age of Ultron and Batman v Superman, Kyle Franzak lays out an apocalyptic vision for a showdown between heroes of both companies that plenty of destruction in its wake.
Some of these kinds of mashups can be jarring, as the styles of the various superhero movies and television series don't exactly complement each other. However, Franzak manages to make it work.
Note: The post has been edited to correct the name of the person who created the mashup.