At the climax of the last book, Myrren Kahliana recovered the powerful artifact called the Dark Heart. With the holy relic in her possession, she sets out for home with her friends Shial, Rahze and Kail. She wants nothing more than to return to her people and her home country in peace, but they have a long and dangerous journey ahead of them. The demonic senkata, whom she took the Dark Heart from, want it back, and they're willing to pursue Myrren and her friends across Caliel to retrieve it. Luckily for them, they discover that Rahze's staff has become able to unlock the power of the Dark Heart, enabling them to use it to protect themselves. Unluckily, there's another change that's coming over Rahze as well, a slow shift in personality that none of them understand.
Even besides the threat of the senkata, Myrren and her friends face obstacles both magic and mundane on their journey: impenetrable wildernesses, cities of thieves and killers, ghostly battlefields, and dark elves who won't abide trespassers. Most unnerving of all is an eerie, unnatural pattern of weather disturbances that seems to be following them.
Back in Vraxor's country, Raine, Arvis, Warde and Leah have finally triumphed over the opposing side in a civil war that tore the nation apart. But the ink is scarcely dry on their treaty when they get the news that the country is under attack again. Kyrian Daimon, the insane messiah so powerful that he's scarcely mortal anymore, has shown up on their border with a huge army and is determined to take his vengeance. What's worse is that he's discovered a prophecy which guarantees him victory if he can silence "the Heart of Vraxor."
Kyrian's immense magical power enables him to destroy every defense that's in his way. Wielding the power of the Dark Heart, Myrren shows up just in time to prevent Kyrian from completely overwhelming Raine and the besieged defenders of Ral Vraxan. Suddenly fearful of facing a power comparable to his own, Kyrian flees.
Myrren is joyfully reunited with her family, but she's furious with Raine, who divorced her at the end of the last book. She accuses him of leaving her for Leah, although both he and Leah protest that isn't the case, but he can't give a satisfactory explanation of why he did do it.
Kyrian returns with his army for a second assault on Ral Vraxan, this time revealing his full power. Even with the Dark Heart, Myrren is outmatched, but manages to drive him off in an effort that almost kills her.
After their near-defeat, Myrren, Raine and all the defenders of the city hold a war conference. They come up with a plan which they're sure will grant them victory. The mood in the citadel is triumphant, until they wake up the next day and find Rahze and the Dark Heart both missing, with Kyrian's war fleet on the horizon, and the senkata closing in...
This novel provides examples of:
- A God Am I: Kyrian's plan is to kill Vraxor and ascend to godhood in his place. Also how Myrren feels when she wields the Dark Heart against Kyrian and his army.
- A Storm Is Coming: Invoked by Chuangzhi, the monk of Rohan. Also applies to the strange thunderstorms that follow Myrren and her friends across Caliel.
- Bavarian Fire Drill: Myrren and co. bluff past some guards at one point by using this trick.
- Bullet Time: Drawing on magic slows one's perception of time. Myrren and Kail exploit this effect to figure out what happened to Rahze's staff.
- Cessation of Existence: Myrren reveals that death is simply not like anything after she's revived, implying this.
- Dangerous Forbidden Technique: All the way back at the beginning of Dark Heart, Arvis warned Myrren against trying to shape magic into two opposing forms at once. She does exactly this in the final battle, activating the Dark Heart.
- Deus ex Machina: Myrren figures out exactly how to use magic in activating the Dark Heart just when she needs to, sacrificing her life in the process. It works, Kyrian's defeated, and she's brought back from the dead too.
- Even Evil Has Loved Ones: Wyre is absolutely devastated at Kyrian's death. Myrren notes with surprise that she really did love him.
- Evil vs. Evil: When the senkata learn what Kyrian is planning, they kill him so as not to be cheated of their revenge.
- FaceHeel Turn: Rahze. And how.
- Fantastic Racism: It's revealed that dark elves aren't evil as a race, unlike in other depictions. Rather, they're simply dark-skinned elves, whom light-skinned elves are prejudiced against, making it basically identical to real life racism.
- Fantastic Nuke: The "arc of fire" of the title.
- Fantasy Counterpart Culture: Kajin is one for China, going by the names of its people and customs like using chop sticks. It even has the equivalent of Buddhist monks. It seems probable Mohender Gosh's home country is one of India, judging by his name and culture.
- Final Battle
- Heroic BSoD: Kail suffers one after being forced to kill his beloved mentor Rahze, who went evil.
- Heroic Suicide: Myrren unlocks the full power of the Dark Heart to banish the senkata once and for all, knowing this will kill her in the process. She gets better thanks to Raine and Kail.
- Heroic Second Wind: Wyre has Kail on the ropes, until she says exactly the wrong thing.
- Hollywood Atheist: Averted. Myrren's doubts about Vraxor finally culminate in her disbelief that he exists, along with the other gods. The series itself has revealed she's wrong, as Vraxor and Nimrod appeared at the start of Dark Heart, while Shial personally has encountered Shayna, though Myrren doesn't know this. Vraxor's holy book however may well be wrong, as she argues. Vraxor and Nimrod having both been gone for a very long time doesn't help to show they exist of course. The author is himself an atheist activist, and thus explores this with more nuance than most, rather than just the usual negative stereotypes.
- Immediate Sequel: Unlike earlier books, this one begins within hours of the previous one ending.
- Kangaroo Court: Mohender Ghosh does not get anything near a fair trial by the Atma Knights' High Council, inspiring Jaelen to bust him out of prison.
- Last Stand: Raine and his friends prepare for one against Kyrian's unstoppable army, but they're rescued by Myrren wielding the Dark Heart. Then Myrren loses the Dark Heart.
- Mass "Oh, Crap!": With Warde's army and the priesthood of Vraxor behind them, Raine and his friends have every expectation of defeating Kyrian in a straight fight. Until he bombards them with magical fireballs that wipe out a fleet of warships in a single barrage.
- Mutually Exclusive Magic: Rahze's staff has become a combination of light and dark magic, something thought to be impossible. This comes in handy, as that's what's needed to activate the Dark Heart.
- Nay-Theist: Kail has concluded that it's pointless to worship any gods, since none have ever answered his prayers to them and give no indication of caring.
- No-Sell: In Kyrian's second onslaught against Ral Vraxan, Myrren hits him with a magical blast that she expects to kill him outright, only to find he's grown so much stronger that it doesn't even touch him.
- Not What It Looks Like: Raine protests to Myrren that he didn't leave her for Leah. She doesn't believe him. Eventually they make up, though he and Leah had fallen in love.
- Outlaw Town: The thieves' city that Myrren and her friends encounter in the Veiled Lands.
- Putting the Band Back Together: Myrren's and Raine's plotlines converge for the first time in three books.
- Screw Destiny: The senkata, being "outside history," have this as an explicit power. They describe themselves as "stronger than fate."
- That Man Is Dead: Rahze says this about his past good self after turning evil.
- The Only One Allowed to Defeat You: Both the senkata and Kyrian invoke this. Each considers vengeance on Vraxor to be rightfully theirs.
- The Social Darwinist: Rahze starts to talk about the strong having the right to crush the weak and such after turning evil.
- Weather Dissonance: All the way back from Palidia, Kail insists that the storms following them feel "wrong," accompanied by vast and inexplicable shifts in the balance of the world. Myrren's use of the Dark Heart is causing it.
- You Are Too Late: Myrren and her friends see Ral Vraxan in the distance just as Kyrian is about to obliterate the city and Raine along with it. They find another option.