Dark fantasy world

Four brothers are in his older brother fantasy novel, the three brothers were now goners. they thought they were still dreaming, but they weren't. why did it had to be his novel, his little brother already know that this world, is filled with demons and other races from another fantasy. why can it be in Solo leveling or Naruto, Saitama or even in marvel world. Lucian was already giving up, and one day something appeared in there dreams. ........................