Amentet. The first book: The kingdom of the dead.

Alexander's measured life is destroyed when he and his pet enter a world where his idea of the essence of man is turned upside down. On the very first day, losing his friend, he sets off on a journey where his only goal will be the desire to survive. And his dog will play a key role in the fate of the local inhabitants, but loyalty to man and friendship will play a bad joke with him.