That could suggest that whatever the danger is, it might have a mystical component, especially considering the events in Daredevil Season 2 and the upcoming Iron Fist series open that door further.
No specific plot details concerning The Defenders have been released yet.
At San Diego Comic-Con, a teaser trailer aired featuring narration from Scott Glenn’s Daredevil character, Stick.
On the other hand, maybe it will just be something scientifically-related, like enhanced technology or more people with special abilities.