Riddick is a criminal on the run from all sorts of people and authorities, but exactly who he refuses to mention. While he's happy to fight other humans, he prefers to try out his exceptional night vision on the lethal critters of the wasteland.
Riddick is a reference to Richard B. Riddick in the Chronicles of Riddick science fiction series. His appearance and abilities are much the same, including night vision. Pitch Black is a reference to the first movie with the character, in which he fought monsters during the night.