It tells the story of Connavar Demonblade from his birth to becoming a leader of the Rigante people. While the Rigante face threats from neighboring tribes, the northern Vars invaders and the expanding southern empire of Stone, Connavar must come to terms with the darkness within himself.
Connavar, the future leader of the Rigante, is born on a stormy night in the settlement of Three Streams. His father, Varaconn, dies in a battle the following day, leaving Connavar an orphan to be raised by his mother Meria and Varaconns's close friend Ruathain, whom Meria marries. Ruathain – the "Big Man" – proves to be a strong father figure in Connavar's life, raising him alongside his own two sons, Braefar and Bran.
Conn has a pleasant childhood, getting into scuffs with his rival Govannan, teaching his crippled friend Riamfada how to swim, and falling in love with the young and beautiful Arian. In order to prepare him for married life, his father sends him to the earth maiden, Eriatha. During this time, Connavar also befriends Banouin the Foreigner, a traveling merchant living in Three Streams. Banouin informs Conn about his homeland, the militaristic and expanding empire of Stone. He warns Connavar of the ruthless efficiency of the Stone army and the ambitions of Jasaray, the Stone general. Sooner or later they will reach the Isles and the lands of the Rigante. Banouin also teaches Connavar the basics of military strategy.
Another feature of Connavar's childhood is a series supernatural encounters with the Seidh, a mystical race of nature-gods who inhabit the Wishing Tree woods. He is gifted a knife after rescuing a fawn that is caught in brambles – later, it is revealed that it was the Thagda he rescued – and when asked what he most desires by the Morrigu, goddess of death and mischief, Conn answers glory. She assures him that his wish will be granted.
As always with the Morrigu, the promise proves to be double-edged. One day, Connavar and his friends are attacked by a bear. In a show of bravery, Conn protects Riamfada and attacks the bear with a knife. The act of heroism almost costs Conn his life, but Ruathain arrives and saves the children just in time. On the verge of death, Connavar's life is saved by the village witch, Vorna, who must sacrifice her powers in the process. The tale of the Boy who Fought the Bear quickly spreads and Connavar becomes a hero, fulfilling the Morrigu's promise. After his recovery, Conn becomes enraged when he learns that his fiancée Arian has married another man.
Months later, Conn accompanies Banouin on a journey to the south to learn more about the civilization of Stone. They travel through the lands of various other Keltoi tribes, some of whom are at war with Stone. Others have already been conquered, or become vassals. When Banouin is murdered on the orders of the Perdii king Carac, Connavar swears to seek revenge by wiping the Perdii people off the face of the earth. After meeting the Stone officer Valanus, he decides to join the Stone army who are currently in the midst of an invasion of Perdii lands. A natural leader, Conn quickly rises in the ranks and gains the favour and interest of Jasaray. He becomes known as "Demonblade", and after killing Carac to avenge Banouin's death, as "the Man who Killed the Evil King".
After the conquest, Conn sees how the Stone army decimate the Perdii, killing many of the survivors, selling the rest into slavery and murdering the children. He has a change of heart, and returns to his homeland with the memory of the Stone army killing innocent infants haunting him. He takes with him Parax, an aging tracker who takes a liking to Connavar.
Conn is welcomed home as a hero. He wastes no time in beginning to prepare his people for the impending war with Stone. Connavar falls in love with and weds Tae, the daughter of the Long Laird, earning him a rivalry with her suitor Fiallach. This turns to mutual respect after Conn proves himself by rescuing Tae during a raid on the coastal town Seven Willows by Vars invaders. Rae's mother, Lady Llysona, is killed in the raid, as well as the brother of the Vars leader Skald.
Tae is killed by accident in an attack on Ruathain, as part of a blood feud with the neighboring Pannone tribe. In a blood rage fueled by revenge, Conn razes the Pannone capital to the ground, burning every home and slaughtering men, women and children indiscriminately. Seeing the aftermath of his carnage, Conn is shocked by the brutality of this demonic side of himself and retreats into seclusion for several months. He is convinced by Eriatha to return to his people and lead them in preparing for the war with Stone. After the death of Tae's father, Connavar is elected to take over leadership of the Rigante.
The Pannones and the Vars, both seeking revenge against Connavar, launch a joint attack against the Rigante. Connavar musters a force to ride out and meet them, including his new Iron Wolves elite unit and led by himself, Ruathain, Govannan, Fiallach and Parax. The enemy force is led by Skald and the Highland Laird of the Pannones. Thanks to superiour military tactics and a reinforcement army led by Bran, the Rigante are victorious. However, Ruathain dies of the strain of his heart after the battle, feeling young again and filled with pride at the bravery of his sons. Conn and the Highland Laird make peace, while Skald is driven back to the sea.
On the mainland, the Stone army has reached the seaport of Goriasa and Valanus looks out over the Isles. The Morrigu appears to him and asks him what he seeks across the water. "I seek fame," he replies at which the Morrigu laughs coldly. "And you shall have it."
- Connavar - Demonblade, leader of the Rigante
- Ruathain - Connavar's stepfather; First Swordsman of the Rigante
- Meria - Connavar's mother
- Vorna - village witch of Three Streams
- Banouin - traveling merchant; ex-general in the Stone army
- Tae - Connavar's wife
- Braefar - Connavar's half-brother
- Valanus - Stone officer who befriends Connavar
- Jasaray - commander of the Stone army
- The Morrigu - Seidh goddess of death and mischief