|BIONICLE Legends 6: City of the Lost|
|Number in series||6|
|Previous book||BIONICLE Legends 5: Inferno|
|Current book||BIONICLE Legends 6: City of the Lost|
|Next book||BIONICLE Legends 7: Prisoners of the Pit|
80,000 years ago... Pridak looks over his army, and is pleased by the sight. He reflects on all of the battles and conquests that have led him to this point. He notices three soldiers of the Matoran rear guard sitting and talking when they are supposed to be at attention, and orders the entire guard to be executed. The rest of the Barraki are discussing their battle strategy for taking Metru Nui and other vital sites in their plan to strike the Mata Nui. Suddenly, a new army bursts into the League’s camp and tears into their soldiers. The army consists of mostly Toa, Rahkshi, and Exo-Toa, and is led by the Brotherhood of Makuta. The Barraki and their armies are quickly defeated, and the warlords brought up before the leader of the Brotherhood army, The Makuta of Metru Nui. He orders them to be killed, but before that happens Botar arrives and teleports them to The Pit. In the aftermath, the armies of the League are broken up and the empires of the Barraki destroyed. Makuta began to wonder about the possibility of replacing Mata Nui. And in the darkness of the Pit, the Barraki rage, and desperately wish for a chance at vengeance.
Now... Reysa, an Onu-Matoran, is out in the airweed field, waiting for the Hydruka to finish harvesting Airweed before the safe hour, the one hour period a day when the underwater predators don't attack, is over. However, he hears a noise that sounds like the scraping of metal. He heads over toward the edge of the rock the airweed field sits on to discover what it is, but turns back after thinking the safe hour is almost over and heads back toward Mahri Nui, only to be killed by a tentacled Rahi.
Kyrehx, a Ga-Matoran sentry, spots and finds a Kanohi mask, (unknowingly, the Kanohi Ignika, the Great Mask of Life). When she goes to show it to the council, she is attacked by animated air weeds by the Fields of Air near the Hydruka Pens, but is saved by Dekar, a Po-Matoran hunter. Kyrehx gives him the mask, and both of them go their separate ways, not knowing that a Squid of the Barraki Warlord Kalmah has spotted them with the mask.
Defilak, his friend Sarda, a Ta-Matoran, Gar, an Onu-Matoran, and Idris, a Ga-Matoran, travel in Defilak's submarine down to the Black Water. They are then attacked from all sides from an organised swarm of Takea Sharks, led by a being named Pridak. After the sub is destroyed, Pridak interrogates them, then takes Defilak, his 'meal' to tell him a tale.
Meanwhile, Kyrehx and most of the other Matoran of Mahri Nui spot thousands of Venom Eels heading towards the city and begin to attack. But before Kyrehx can launch the attack, she is taken by Carapar to Takadox. Takadox hypnotises the Ga-Matoran and forces her to tell him where the Ignika is. When Takadox hears that Kyrehx doesn't have the mask, he tells Carapar to go and tell Ehlek to stop commanding his Venom Eels to destroy the city, in case the mask is still inside the city. Dekar then finds out that the Kanohi mask he possesses is the Kanohi Ignika, Mask of Life, and realizes that it will keep healing any nearby attackers, he then decides he is going to destroy it.
Pridak tells Defilak about the League of Six Kingdoms, his and his fellow Barraki's once known armies. Kalmah and Mantax then find out that Dekar has the Ignika and begin to chase him whilst he attempts to destroy it in a nearby cave which causes the Ignika to create a guardian, a 300 foot-long Venom Eel. Carapar manages to force Ehlek to stop launching attacks on Mahri Nui in case the mask is still there. The two then meet Brutaka, a fallen Order of Mata Nui member who then commands that they take him to the Ignika or perish. Kalmah, Mantax and Pridak begin to fight the 300 foot-long Venom Eel which has destroyed part of Mahri Nui's upward pointing stone cord, and the Barraki are repeatedly being defeated but they are not going down.
Then, Brutaka, Ehlek and Carapar arrive. Ehlek and Carapar are hoping that Brutaka will rid them of their leader but Pridak suspects this and Brutaka does not. Brutaka then sees Dekar with the Mask of Life and goes over, claiming that he is an Order of Mata Nui member and Dekar needs to give the mask to him. Dekar refuses, saying 'If you were meant to have the mask, you would have found it first, wouldn't you?,' Pridak, finally annoyed, commands Kalmah's giant squid to drag Brutaka off to the Black Water. As Pridak corners Dekar and takes the Ignika, it sets off a bright light, which begins to envelop the whole of Mahri Nui.
- Ehlek's eyes are seen as red on the cover of the book, when they are actually blue. In addition, his Squids are yellow-orange and not black.
- This is the first book not to feature any Toa in it.