While the first Justice League movie came out late last year, for many "fans" this was the first time they've seen Justice League outside of a TV series. However DC has been making pretty good animated films for many years now. So while the silver screen movies are just getting started, the animated ones are getting closer to pretty fresh story-lines from the comic books. Justice League Dark is about a more paranormal side of the DC universe. The only character I recognized in the roster (Batman technically isn't in the Justice League Dark) was John Constantine since there was a movie many years ago with Keanu Reeves.
The story begins in a typical day. A woman gets into a car accident, but she seems a demon get out of the car and try to get into her. She panics and sees demons everywhere. She runs over them, and her car is eventually picked up into the air by Wonder Woman. The Amazonian warrior is rather pissed at the woman, as the "demons" were really normal people. Superman catches a man who almost killed his family over the same kind of thing. When more stories like these come up all over the world, they start to realize this is no mere coincidence of crazy people. The Justice League decides this is in the realm of magic, and Batman decides to find people who are better suited with the paranormal, like ghosts and demons.
Batman tracks down a magician named Zatanna who knows John Constantine pretty well. Both John and Zatanna are pretty talented at magic though she is more powerful with attacks and John is more talented with more complicated spells and techniques. John however is not good at keeping friends, and has to use his "charm" to convince his fellow paranormal people to help figure out who is behind this dark magic and put a stop to it.
Overall while it's not the best of the original DC animated films, I will say it's one of the better ones. I think of the one of the reasons is that it gets pretty grim and dark. Another is that we've seen countless stories with Batman, Superman, etc, but most of these heroes we have not so that's more original territory. It's a little rough around the edges, but it's funny when DC animated movies are sometimes better than the big-budget DC movies.