To protect her village from the invading army of the Divine Empire, Gaelic Priestess Riannon agrees to be the human sacrifice needed to revive the Demon Lord, Arawn.
Arthur finds where Riannon has been taken, but cannot kill Drwc or else his sister will die.
The villagers are preparing to leave for Albion, but their ship has been captured by the Imperial Army.
The tribe heads for the elfin city of Avalon, where they meet Limwris and Ermin.
Arthur and Arawn try to hunt for food, but find they are in another tribe’s territory.
The girl Morgan’s flute has summoned is Llyr, of the faerie tribe.
Morgan and Llyr, frustrated with their duties, complain to Limwris about their situations.
Gaius learns that the Senate’s own security force, the Rublum, has been dispatched to destroy the Gaels. When Morgan spots their ship approaching, Ogam warns that they are an elite force and should not be underestimated. Arthur ignores his advise.
Octavia is insubordinate to her superiors and stumbles upon a secret. As punishment, she is sent to the mines, where she is under the orders of an old rival, Lidia.
Arawn is concerned with how the rest of the tribe will react to Octavia’s presence in the castle.
The Rubrum learn the location of Avalon Castle, causing some of the Gaels to suspect that Octavia is a spy.
Riannon is captured by Gaius and forced to heal his Imperial soldiers. Arthur wants to leave immediately to rescue her, but is cautioned by Arawn not to be hasty. Arawn is able to locate Riannon and determines that she is safe for the moment.
Gaius leads another attack on Avalon Castle. Arawn decides to try to form an alliance with the Brigantes Tribe, their neighbors. They go to meet Taliesin, but learn that the Brigantes are in disorder. The chief has died, but without naming a successor.
The tribe is running low on supplies and funds. Arawn takes Arthur and a group to his tomb to collect some of the treasure there.
As the Imperial forces gather around them, Arthur makes the impulsive decision to lead a preemptive strike. As the group passes through the forest, they cross into an area under the enchantment of the Gravitas.
With her husband injured and her brother gone, Riannon must take command of the tribe and repel the invasion of Avalon by Gaius’s forces.
The siege of Avalon has continued for three days. Riannon has led the Gaels in repelling Gaius’s troops. Arawn’s wounds have healed, but he has yet to regain consciousness.
With the Gael’s cornered and their last lines of defense crumbling to Gaius’s forces, Arthur and the Brigantes arrive. Arthur calls out to Arawn, who finally awakens and joins the fight, giving his people a new strength and resolve.
The Divine Empire is in ruins, and with the High Council dead, it falls into chaos. A new foe, White Spirits, emerge to take advantage of this vulnerability - or so they appear.
The 12 Angels in heaven have placed their hopes for the future of humanity in the hands of Merlinus when they give him charge over the new Angel.
Taliesin gets a glimpse of Arawn’s past, which reveals the actions he, himself, must take.
Taliesin’s sacrifice has stopped the enemy in their tracks, but Arawn knows that they must act swiftly to defeat them. Their only hope is to retrieve the sword Dyrnwyn.
Together again at Palladium, Arawn and Arthur lead the others into the tower.
With the Gravitas enchantment hindering their defenses, Arawn and Ogam split off to find the source.
Having already destroyed both the Obelisk and Resurrection Cauldron, the group defeats the army of golems and proceeds to the top of the tower.
Arthur and Arawn are unable to defeat Merkadis. It appears that only Riannon possesses the power to stop him, but she cannot remember how. Her brother and husband continue to fight, giving Riannon the chance to remember what she must do to save them all.