Invictus

Invictus

Out of the night that covers me,
Black as the Pit from pole to pole,
I thank whatever gods may be
For my unconquerable soul.

In the fell clutch of circumstance
I have not winced nor cried aloud.
Under the bludgeonings of chance
My head is bloody, but unbowed.

Beyond this place of wrath and tears
Looms but the Horror of the shade,
And yet the menace of the years
Finds, and shall find, me unafraid.

It matters not how strait the gate,
How charged with punishments the scroll.
I am the master of my fate:
I am the captain of my soul. 

William Ernest Henley

The metaphorical concept of Invictus is that darkness is horrible and evil or something like that. It comes from seeing darkness as being something to be scared of. The Doctor explains this as caused by the presence of Vashta Nerada. However, a seemingly more likely explanation is that people rely primarily on sight to experience the world and in the dark people have difficulty seeing and so it disconcerts them. Add to this the presence of predators that hunt at night and human imagination and one gets fear of the possibilities of the dark. Evil is something that people have come up with and something people are often also afraid of. Linking these two frightening things together isn't that difficult.

The metaphor is reflected hugely in culture, especially in fiction. In films when something is supposed to be evil, hated, depressing, etc. the world is often made dark. In fiction in general the evil antagonist often is dressed in black or accompanied by darkness. The metaphor can even be seen in language where the 'dark' has come to be synonymous with negative feelings such as hate or misery.