SPOILER WARNING: The following article contains spoilers for Justice League, in theaters now.
DC Comics' Parademons first appeared in 1971 when Jack Kirby introduced readers to the New Gods, alien beings from twin planets, New Genesis and Apokolips, who were embroiled in an eternal cosmic war. The latter, the home of Darkseid, was where the Parademons were being genetically engineered in laboratories to serve as mindless soldiers. In their earliest iterations, they were harvested and bred from human stock from Armagetto, one of the most blighted districts in the fiery wasteland of Apokolips, in order to then be used as the first wave of attack whenever Darkseid decided to invade planets, usually Earth.
Fans of the DC Extended Universe first glimpsed these Apokoliptian foot-soldiers in 2015's Batman vs. Superman: Dawn of Justice in the Knightmare sequence. In this scene, Bruce Wayne got a dark glimpse (we're not sure if it was a dream or a premonition) of Earth under siege from the Parademons, who were surprisingly part of an attack led by Superman.
We saw Darkseid's Omega symbol burnt into the ground and most notably, Superman, seemingly under Darkseid's influence, on the cusp of killing the Batman. While this was just a cameo, in Justice League, they play a bigger role as Steppenwolf's regiment when he comes to Earth for the three Mother Boxes he needs to enslave the planet in Darkseid's name.
But in Zack Snyder's third film in the DCEU, as Wonder Woman explains Steppenwolf's history with Earth, we see that it's a bit different in terms of how the Parademons come to be. She reveals that the villain invaded in the past, intent on terraforming Earth into a wasteland similar to Apokolips, only to be met by a unified front consisting of human warriors, Amazons, Atlanteans, Greek gods such as Zeus and Ares, and yes, a Green Lantern.
In the midst of battle, when Steppenwolf was able to harness the combined power of these Mother Boxes, in a process known as Unity, the warriors who opposed him didn't die when they were killed or harmed, but they were genetically converted into Parademons. It's similar to the Parademon conversion process in the New 52 where they were created through the reformatting of genetic material collected from the remains of the different species from all over the multiverse.
In addition to the dead remains of these beings, in the New 52, the conversion was also done by scavenging living creatures, snatching them up and taking them to conversion facilities where technology was then used to program them to serve Darkseid. This process was also similar to that in the Justice League: War cartoon, but in Snyder's movie, it's more direct scavenging in the field of battle, with the conversion happening almost instantaneously like a rabid virus infecting the victims.
The movie's insect-like Parademons then utilize laser blasters, super-strength, a high tolerance for pain and also, battle-armor that grants them the ability of flight, just like in the books. What's interesting is that they also self-detonate under duress, as seen when Batman captured one at the start of the film.
One key aspect of these soldiers in the movie is that Batman recognizes they're drawn to a certain sound frequency, which he then replicates through his Batmobile speakers to manipulate them into a trap as part of the League's toppling of Steppenwolf. Another noteworthy is the Parademons' nature, where they feed on fear as they hunt their prey. This leads to them being called 'nightmare-creatures' in the film, which ironically ends up backfiring on Steppenwolf whose fear ends up being fed upon by the Parademons when he realizes he's literally out of his league in battle.
Now in theaters, Justice League stars Ben Affleck as Batman, Henry Cavill as Superman, Amy Adams as Lois Lane, Gal Gadot as Wonder Woman, Jason Momoa as Aquaman, Ezra Miller as The Flash, Raymond Fisher as Cyborg, Willem Dafoe as Vulko, Jeremy Irons as Alfred Pennyworth, Diane Lane as Martha Kent, Connie Nielsen as Queen Hippolyta, Amber Heard as Mera and J.K. Simmons as Commissioner Gordon.