So... you remember the Arai? The spirits of curses... They have the means to use them against the gods, they confirm that they only did not go against Iapetus since he had his mind wiped... with that in mind, there has to be millions of curses against the gods.
And Nemesis and other children of Nyx can change Fate and Destiny... which the Arai are...
So what would happen, hypothetically, if the build-up was too much... and something released. The curses gathered together and changed Fate, creating a child that would fulfill the curses in some fashion. Like the curses themselves were reincarnated into a child...a weapon.
A child born to the curses, with no parents... to the curse that specifically says something along the lines of 'Let the gods feel they cause' And that child is born and found and eventually is thought to be a demigod. They are sort of a reflection of Percy and are bound to him even, they might even meet him before. Doesn't matter...
I am unsure of their powers, but what I do know is that they slowly grow stronger and stronger, absorbing the curses of others until the day they can turn against the gods and fulfill their destiny and the curses they were born to.
Thoughts, ideas, suggestions...