Nyctophobia - a simple word. Nycto means darkness (from Nocturnal) and phobia meaning the fear of. Compounded together, it means 'the fear of darkness'. But not only that, it is the fear of what will happen to oneself in the darkness. This word is savored in these times by the very thing it talks about.
The Darkness is not a being, it is not some sort of diety or unseen object or being that strikes fear in the hearts of the free world, alright so it does a bit; nay the Darkness is what happens before the forces that serve under it sweep over the land in a crushing, unrelenting push. Those few who have seen it dare not speak of it, for it steals souls even if the land hasn't been in turmoil yet.
The Darkness is simply the comming of, well, darkness. The sun seems to be blocked out by the night, even in midday. For this there are many explinations. One is simply because the forces of Darkness cannot fight under the brightness of the sun, it blinds their minds and shatters their fearsome presence. Another reason is that it takes much from the land it occupies its will to fight and to resist the comming of the darkness.
Can the Darkness be stopped? Many say no, few say yes and even fewer say most definaetly yes. But the Darkness, says most definaetly no.