Sisters by the Styx Book 5: The Blood of Poseidon

Having a serial killer on the loose would be enough trouble for the summer, but being a demigod means the Fates are always going to go above and beyond when it comes to making your life difficult. When one of their own is taken, it's a race to mount a rescue mission with communications on the fritz, the Oracle silent and the threat of being too late looming over Camp Half-Blood.