Lets go back in time, to when Quinlan was known as "Quintus Sertorius", the gladiator, the fifth hybrid. As he and Cori hunt River, a new breed of hybrid vampire, his story is told. He's made an offer to trade his immortality for a mortals life. Live out the rest of his days in a human form, or keep his immortality and watch the one he loves wither and die, never able to truely possess her.
River, the master's bite has transformed him, inhuman speed, strength, mind compulsion, and an insatiable thirst, are the gifts/curse endowed upon him. He can change strigs into humans and humans into hybrids, and giving the gift of immortality to another, he wants to change Cori. What good is a life of immortality, if you can't share it with the one you love?
The hunters have now become the hunted... Eph's son was taken by the Master, and murdered by Quinlan and Cori, thats the way he sees it. River could have changed his son, given him his life back, but they took that away from him. Recruiting Dutch and Fet, they're on the hunt for a murderess, a half-strig and a vampire hybrid. The war ends with all three of their deaths.