In Justice League Dark: Apokolips War, Darkseid nearly eradicates the Justice League. He was able to do so by spying on their plans through Cyborg. How is he able to do this?
On this wiki of Darkseid, it lists all the movies he is present in. I've read the page and the synopsis of the movies he is in and could not find how he is able to see what Cyborg is seeing. In the movie's wiki it states that Darkseid is spying through Cyborg's Apokolyptian technology. Why is he able to do this?
Via Motherbox and the rest of the Apocalyptian technology embedded inside Cyborg.
What is Cyborg and how was he created?
Victor Stone was a son of a scientist researching technology from Apocalyps. There was an accident and the technology was fused with him. Victor's father did extensive surgery on him to save his life, installing technology he didn't really understand well.
It is all described in Justice League: War movie.
The technology inside Cyborg is originally from Apocalyps, including the Motherbox. It is the difference between creator of technology and user. Cyborg is user, Darkseid is a creator. He has access to functions Cyborg doesn't even know about.
