Here is the official synopsis from the press release:
When innocent civilians begin committing unthinkable crimes across Metropolis, Gotham City and beyond, Batman must call upon mystical counterparts to eradicate this demonic threat to the planet. Enter Justice League Dark, reluctantly led by the Hellblazer himself, John Constantine. Like Batman, Constantine is a cunning, often cynical loner who is the best at his chosen profession – but quickly realizes the sinister forces plaguing the planet will require help from other supernatural alliances. Forming a new “league” with sorceress Zatanna, otherworldly Deadman, and Jason Blood and his powerful alter ego Etrigan the Demon, this team of Dark Arts specialists must unravel the mystery of Earth’s supernatural plague and contend with the rising, powerful villainous forces behind the siege – before it’s too late for all of mankind.
Actor Matt Ryan, who set the standard for the role of Constantine on the Warner Bros. live-action television series, returns to the role in animated form alongside Jason O’Mara (Terra Nova, Agents of S.H.I.E.L.D.) as Batman, Camilla Luddington (Grey’s Anatomy) as Zatanna, Nicholas Turturro (NYPD Blue) as Deadman, Ray Chase (Final Fantasy XV videogame) as Jason Blood/Etrigan, Roger R. Cross (24, Arrow) as John Stewart/Swamp Thing, Jeremy Davies (Justified) as Ritchie Simpson, Rosario Dawson (Daredevil, Sin City) as Wonder Woman, Jerry O’Connell (Stand By Me, Crossing Jordan) as Superman, Enrico Colantoni (Flashpoint, Veronica Mars) as Felix Faust, and Alfred Molina (The Da Vinci Code, Spider-Man 2) as Destiny.
Justice League Dark is directed by Jay Oliva (Batman v Superman: Dawn of Justice, Batman: The Dark Knight Returns). Story by J.M. DeMatteis and Ernie Altbacker (Green Lantern: The Animated Series) and teleplay by Ernie Altbacker. Sam Register is Executive Producer. Benjamin Melniker & Michael Uslan (The Dark Knight trilogy) are co-executive producers. James Tucker (Batman Bad Blood, Batman: Return of the Caped Crusaders) is Supervising Producer.
The film will be available from Warner Bros. Home Entertainment on Blu-ray Deluxe Giftset, Blu-ray Combo Pack and DVD on February 7, 2017, and on Digital HD starting January 24, 2017.