The Bones of Makaidos is the fourth and final book of the Oracles of Fire series, and the eighth book overall.
As The Bones of Makaidos begins, a wall of fire protects the inexperienced villagers of Second Eden from a planned invasion of dragons and Nephilim, but how long will the flames last? Billy, Walter, Ashley, Elam, and the faithful dragons help the people prepare, but they are woefully outnumbered and will have to go back to Earth and recruit the humans who have the ability to revert to their former dragon states. In search of aid, Billy escorts Acacia, and Oracle of Fire , through a dangerous volcano portal.
While in hades the refugees are attacked by the caitiff. But during the defense Bonnie and Sapphira are closed off, they manage to find a way to open the dangerous river of fire portal and escape through it. The portal now leads to the valley of souls a place from which Abaddon sanctions all resurrection. Bonnie and Sapphira are spit out there and after a confusing conversation with Abaddon begin training with the famed Joan of Arc.
With Hades and Earth locked in a catastrophic merging of their two realms, Arramos, evil incarnate, plans to use Bonnie to add Second Eden to the convergence, therby drawing his forces to Heaven's Gate, where he hopes to gain access to divine authority.
Arramos has two secret weapons: Sir Devin, the greatest of all dragon slayers, lies in wait to destroy all dragons and their offspring , including Billy and Bonnie; and Mardon, a brilliant scientist, knows the secret to the Oracles' indestructability, and he has devised a way to steal it from Acacia, therby draining her strength. Meanwhile, Semiramis, Mardon's mother, comes to Second Eden and provides Billy with a wealth of information about Arramos's plans, and with every subsequent challenge, her words are proven true. But can she be trusted?
With mysteries abounding and an ultimate battle looming, every decision could mean the difference between survival and catastrophe, and only a prophesied sacrifice can stop the onslaught of evil. but who will be the sacrificial lamb?