Hugh and his friends survived the City of Ithos but Hugh was badly injured in the process. Kanderon's enemies are coming for her, the Splinter, and the Tongue-Eater, which she's had since she originally used the Splinter. She keeps it in the Skyhold Vault with the other things that should never be used. That is, after all, the true purpose of the Librarians Errant of Skyhold, to collect dangerous items and remove them from the world. Back at Skyhold they are preparing the defenses but with most of the Council killed by Bakori there is one wounded councilor, one admitted traitor, and Kanderon who has been gone dealing with Ithos. They may be weaker than usual but Kanderon built Skyhold with strong defenses and she has strengthened her alliance with the other Powers of the Erg desert even if many of them cannot leave their territories.
Since Alustin and Audric are busy preparing for the eminent siege Hugh and his friends have new responsibilities. Hugh is asked to mentor a group of students which seems ridiculous since his schooling was never the "usual" but they turn out to be warlocks like himself. Talia's family has come to Skyhold to help with the defense since the school managed to help Talia use her magic and they love a good fight. Since her clansmen are known as the world's greatest fire mages they are most welcome. When she's not trying to keep them from causing trouble she's busy mooning over Hugh. She loves him and he's oblivious. In fact he's the only one who doesn't know. Sabae is still shocked to discover that her grandmother has allied with Kanderon and was working with her to manage the Ithos situation. Still, there is a feeling of oppression as they wait for their enemies to make their move.
The plot of this fifth book is all in the title. There will be epic battles between various Powers. The Empire manages to find some very strong allies, including the solar dragon Heliotrope who is considered the greatest power to call their world home. Alustin's arch enemy is back with her warlock minions and their sentient weapons. There is an enormous hydra. Kanderon's allies are not without power. The great storm dragon Idris comes with her many many children. The gorgons come because the reclusive warriors were once allied to the Lord of Bells, the Lich who Alustin's family once owed allegience but was destroyed by the Empire. Godrick's father proves himself to be more than simply an archmage or Lesser Power. The former mercenary mage wrecks havoc on the Empire's Powers. Talia's clan defies those who think of them as simple barbarians by facing off against Great Powers on their own. Hundred's of the Empires forces fall afoul of the wards designed by Hugh's instructor Leila. Then there is the greatest Power of Skyhold, the Crystal Sphinx herself, Kanderon.
In the midst of this chaos Hugh and his friends refuse to hide with others in Skyhold's bunkers. After their initial position is overrun they take to the halls shoring up the defense where they can. Their ascent to the council chamber and final defense of the Vault is full of sorrow as characters they know heroically giving their lives to defend Skyhold. Then, at the climax of this epic battle, Hugh is blindsided by an unthinkable betrayal.
The Siege of Skyhold is a wild ride of highs and lows that shouldn't be missed.