As other reviewers have noted, this is a direct sequel to Robert E. Howard's seminal tale "Worms of the Earth", written by a sword & sorcery giant of the third generation: Karl Edward Wagner. As others have also noted, this is about as good as pastiche gets -- and in this case, that is very good.
Bran is one of Howard's most interesting characters; a Pict watching his people decay into savagery, even a subhuman brutishness. He has much of Kull's introspection, but a greater melancholia and an anger and rage at his people's eventual fate that is barely kept in check. Bran did not receive the canon of tales Conan, Solomon Kane or even Kull did, but the ones he did are *very* good and fit into a larger body of Howard's work that chronicles the constant rise and fall of the First Race of Man, and their eventual doom to retreat into the hidden places of the word, there to be remembered in stories of dwarfs and elves -- a parallel doom to the earlier Serpent Folk, the Picts' eternal foes through the long eons, and whom they drove under the earth centuries before Bran's era.
These themes of rise and fall of civilization, of physical and mental degeneration, of hidden histories -- all are baked into Howard's work, but his tales of the Picts (and Bran) range from when he was 16 until his death at 30, and when you read the stories together, there's some loose seaming in which Howard's evolving ideas and backstory don't always neatly fit. Wagner here not only writes a compelling sequel, but uses it as a chance to rectify and smooth over those bumps. In a long chapter that is straight out of Howard's beloved trope of past life or racial memory (and when we are talking about humans, serpent people and beast men, I think 'race' is a fair term!) he creates a syncretic history of the Picts and their war with the Serpent Folk that draws purely from Howard, in a way he would have written it.
So that's a nice touch, and Wagner does a great job of writing in a "Howardian" style without resulting in mimickry. His Bran also feels like Bran, and the expansion of Bran's sister, Morgaine, who is only mentioned in one tale, is nicely done.
Thus, we have some of the key checkpoints for good pastiche:
1. Does the author understand the character? Yes.
2. Does he write convincingly vis-a-vis the original material? Yes.
3. Does the pastiche have a logical place vis-a-vis canon? Yes.
Now the big one:
4. Is the story any good?
Yes! It is very blood and thunder, and fits the mood and fulfills an ominous prediction at the end of "Worms of the Earth" in a way the original author might have gone, had he continued the cycle of stories.
Some like pastiche, some don't. If you do, this is a great example of how it can be done well.