Hidden Darkness (Chronicles of Kerrigan)

The enemy has a new face now, Jonathon Cromfield. Except he's not really new, is he? Cromfield has a plan, one that includes Rae whether she likes it or not. Rae is terrified by the idea that she can't die. Everyone she'll ever know will move on without her and leave her all alone. Except Cromfield. She'd rather die than be with him. The team of Rae, Devon, Julian and Molly must race around the world to find the hybrids before Cromfield does, they have to figure out what his serum really is for, and take it back to the Privy Council all before Cromfield can stop them. Immortal or not, Rae doesn't have a second to waste.