tl:dr Cults of the Blood Gods reintroduces the Clan of Death to V5, gives details on vampiric cults, provides new mechanics (loresheets, powers, predator types, etc), and gives a story featuring the Clan of Death to tell.
As a fan of Vampire: the Masquerade lore, Cults of the Blood Gods is easily one of my favorite V5 releases to date. This book opens with a well written fiction giving just a hint of how the Hecata came to be formed, with several smaller additional pieces after the introduction. Those first pieces gives a good taste for what is to come in the rest of the book.
I'll be honest, none of necromany based clans or bloodlines in prior editions ever really caught my eye, with the possible visual presentation of Baron Samedi. Other than using one as an SPC as a Storyteller, I hadn't been interested in playing one. Cults of the Blood Gods spurred my interest in playing a character of the recently formed Hecata clan. The merging into one clan creates many interesting roleplaying opportunites, whether it be between the different factions of the clan, the opportunity to play through what happened within the clan once the Family Reunion takes effect, or how they present themselves to outsiders under the Hecata banner. I like how Oblivion is presented and the steps taken to differentiate Oblivion Ceremonies from Blood Sorcery Rituals.
While vampric relgion has been addressed in other V5 books, none have done so in a way that has felt playable. Cults of the Blood Gods dives deeper into the subject with multiple cults, some new to Vampire:the Masquerade, some from previous editions that are presented in a way that they still feel new. For the first time in V5 the Bahari, Church of Set, and others feel playable with digging through multiple books from prior editions. In addition to cults for vampires, the book also provides cutls for mortals (that are run by vampires) and a section on how to create your own. Again, the topic of mortal cults have been covered in older editions of VtM however in this book the subject is covered in a way that gives more detail into how cults function. I don't plan on building my own cults, but this book gives you plenty of information on how to do so.
The book ends with Styx and Bones, which gives a story that can be run highlighting the clan of death. It is a decent enough story, I just personally would rather explore other aspects of lore and metaplot presented in this book. If someone asked me to run it as an ST I would, but it is not something I would run on my own.
In addition to the above, Cults of the Blood Gods sprinkles in new machanics in the way of Loresheets, Discipline Powers and Amalgams, Rituals and Ceremonies, Predator and Coterie types, and more. The art through out the book is amazing. I would highly recommend this book for anyone who plays V5.