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
Darkness here is associated with evil (“the pit” as in the pit of hell), which covers the world from pole to pole (North and South and everywhere in between). 
 ‘Pole to pole’ is a reference not only to the external world, but could be a metaphor for every aspect of the poet, physically, emotionally and spiritually. 
 The imagery here is biblical, maybe alluding to the notion that we were all born sinners ‘— out of the night that covers me’: night being associated with original sin and the Fall of man.  However, the following line makes it clear that the speaker is not a specifically alluding to the conventional Victorian Christian beliefs.
Invictus