Falling Between Two

After Annabeth's sudden death six years after the events in HOO, Percy locks himself away for more than a year, swallowed by his own sorrow. He reappears without warning at Nico di Angelo and Will Solace's shared doorstep, asking to visit. He visits frequently after that, at least twice a week, and everything seems to be going back to normal, until Nico's health plummets.
He can't stand or walk on his own, and is in a constant wave of nausea, headaches, and vomit. Yet there is nothing wrong with him that even Apollo- the God of healing can sense.
He will surely die without treatment, but without a cause, what can be done for him? Will is prepared to do anything to save his best friend's life.