Anon's Q said Darnkess - not Darkness.
A book:
Three Days of Darnkess by Glen Aitken
John Staggar must fight his way through the city to save 5 year old Susan Riddle during three days where hell reigns on earth.
Note that the book title which is printed on the cover while you found it when using Darnkess.👀👀👀
