Percy Jackson And The Fates

Percy Jackson died while saving the world from Gaia. A year later, the Strings of Fate are stolen, and the seven take on the quest from the gods. But an unexpected man shows up and he calls himself Enard, The Blood Dancer. Who really is this man and who took the Strings of Fate?
(Hint: Enard is someone else disguised)

Starts at the Blood of Olympus.