Upon arriving in Hallandren, Vivenna meets with Lemex, one of her father's spies in the city, but he has taken ill and dies shortly thereafter — though not before bequeathing his large sum of BioChromatic Breath to her (which is considered heretical by the Idrians).

Vivenna joins up with Denth and Tonk Fah, mercenaries that were under Lemex's employ, and together they begin making guerilla attacks against Hallandren's supply depots and convoys that will hopefully give the Idrians an advantage in the seemingly inevitable war.

However, Siri believes that the priests are secretly plotting to kill her and the God King if she produces an heir, and fears that Hallandren will soon launch a war against Idris.

Siri finds a potential ally in the unorthodox god Lightsong, who is plagued by nightmares of war and is struggling to discover his purpose.

Back in the city, Vivenna discovers that Denth and Tonk Fah are not working for her but against her, having been hired by an unknown third party to instigate the war with Idris, and she barely escapes their custody with her life.

After hiding and living destitute in the Idrian slums of Hallandren for weeks, Vivenna is taken in by Vasher, a mysterious man who can use his BioChromatic Breath to Awaken objects, an art at which he is incredibly skilled.

However, Vasher is captured and tortured by Denth, who is revealed to have been working for the God King's Pahn Kahl servants, who are trying to incite war between the Idrians and Hallendren so that they can take the city for themselves.

The Pahn Kahl, having gained the Commands to control the city's undead Lifeless army, send them to attack the Idrians and start the war.

Meanwhile, Vivenna uses her own budding BioChromatic powers to break into the God King's palace and free Vasher, who kills Denth.

While Siri and the God King begin a new rule and life together, Vivenna joins Vasher as he sets out on another quest to a distant land.

"[7] In the early chapters, Sanderson begins to show a swap in the roles and attitudes of the main characters Siri and Vivenna.

That story is absolutely up to Sanderson's very high standard, with political intrigue, carefully differentiated cultures and believable human motivations.

"[4] Michael Moorcock also praised Warbreaker, saying: "Brandon Sanderson has written a heroic fantasy depending on originality of character and plot.

He finished, "Not only has Sanderson drawn a freshly imagined world and its society, he has also given us a plot full of unexpected twists and turns ...

"[12] SFFWorld called the book "well-wrought, intelligent, and at times, surprising – one might say a conspiracy novel with slight hints of 1984 wrapped in a wonderful fantasy package."

[16] Aspects of Warbreaker - particularly two of its characters and the sword Nightblood - have appeared in some books in The Stormlight Archive, which shares a universe in Sanderson's linked 'Cosmere'.