The 'Nos (night)' gene causes one of two things to happen; the first and most common effect for the 'complete' Wyllt races is that, during times of great distress, anger, depression or negativity, the creature will revert to a more primal form of its race becoming incredibly powerful but completely primal.
It is very rare for the transformation to last more than a day but occasionally the creature can be lost forever. The second transformation is a permanent version of the ancestral Wyllt form, depending on the strength of the creature, said infected can retain their sanity and continue to live normal lives in a way.
These are known as Nos Pur (pure night) and can infect 'humanoid races' to create beast mixes or Greadur who can become either transformation.