This is book two of the Arts of Dark and Light series. Book one, A Throne of Bones, is required reading before reading Sea of Skulls, as are the various novellas like A Magi Broken, The Last Witchking, The Wardog's Coin and if possible the novel Summa Elvetica: A Casuistry of the Elvish Controversy (Arts of Dark and Light Book 0), although the latter is not necessary for enjoying the series. As Goodreads describes it (because I can't suitably summarize the complexity), "Day brilliantly re-imagines ancient Rome with warlike neighbors including orcs, goblins and trolls playing the roles of nomadic mongols, marauders from Gaul, and more generally, the anarchy that threatened and eventually enveloped the Roman empire during its fall in the 5th century. Except Day's Rome also features magic. And a neighboring human empire that behaves much like feudal England. And a fading Elvish empire reminiscent of the Greeks. And interesting god-like villains who serve as agents of change, but may not be villains at all." It is a LOTR-like story that has action seemingly on every page, and very little exposition. Day doesn't ever tell, he shows. The characterizations are complex and deep, even for the orcs and goblins. The main POVs include every species and they interact continuously instead of rarely like in LOTR. The orcs, trolls and goblins have amassed a combined invading army of one-hundred fifty thousand and the book features some battles but has more of a set-up for future books theme. It is a much broader in scope book than book one, which focused much more on the Roman Empire-like empire of Amorr, which is ruled by a Senate predominantly made up of the members of the Houses Martial, which field the legions for their defense as well as their frequent conquests. The other human country is Savondir, which is a monarchy straight out of the middle ages. Rather than being at war again, the two countries have a tense partnership to try to fight off the invading army. The Elven Kingdom is made up of three provinces each with a king, and there is a High King who rules over all. Again the usual conflicts of the past with the humans are set aside in an attempt at a unified defense, but the elves have neglected their armed forces in favor of esoteric pursuits, primarily magic. Magic is also a major aspect of Savondir but is strictly forbidden and feared in the Christian-like Amorr empire. I read the first book, A Throne of Bones, when it first came out and have been longing for book two, which is even better. I consider this to be one of the five best epic fantasy series being written, along with Traitor Son (The Red Knight), Lightbringer and Song of Shattered Sands (Twelve Kings of Sharakhai) and Immortal Treachery. I'm sure few if any are aware of this great series, but I say without fear of disagreement that everyone who loves epic fantasy will love and be blown away by this wonderful world.