An animated feature film starring the Justice League Dark, which here consists of Constantine, Zatanna, Deadman and Jason Blood.  While Swamp Thing features prominently in the film’s promotional material, he only appears in two scenes.  Black Orchid also has some minor appearances and it is explained that she is the personification of the House of Mystery.

Cast & Crew
Director: Jay Olivia
Writers: J.M. DeMatteis and Ernie Altbacker.
Roger Cross as the voice of Swamp Thing

Summary:
The Justice League investigate a series of unusual homicides and suspect there may be dark magic at work.  Batman gathers Zatanna, Deadman, Constantine and Jason Blood/Etrigan, and they are tricked by Constantine’s friend Ritchie into accusing Felix Faust.  The group soon learns that Ritchie is being manipulated by Doctor Destiny, who seeks the Dreamstone.  The Justice League Dark, aided by Swamp Thing, battle Destiny and fend off a crazed Justice League.  Destiny is killed but not before managing to separate Jason Blood from Etrigan, and Alec Holland from Swamp Thing.

Trailer:

Release:

Ultraviolet/Blu-ray/DVD (75 min.), Warner Home Video, 2017. A notable ‘special feature’ is The Story of Swamp Thing (18 mins.), which includes interviews with Len Wein and Kelley Jones (amongst others) about the character’s origins, Wrightson’s art style, the arrival of Alan Moore, and the 2016 miniseries.  There are also comments by some of the filmmakers about Swampy’s role in the JLD movie.

Sequel:
There was a loose sequel released in 2020 called Justice League Dark: Apokolips War. The JLD lineup here is even further removed from the comics. Constantine and Etrigan join Raven, Damian Wayne and a de-powered Clark Kent to lead a resistance against Darkseid, who has enslaved Earth. Swamp Thing and Black Orchid only appear briefly in a couple of scenes, and Roger Cross again provides the voice of Swampy.

Further Links:
IMDb – production details, photos, reviews.