He-Who-Must-Not-Be-Named

Percy Jackson's fatal flaw is loyalty, and a certain grandfather of his wishes to exploit that fact.

The war with Gaea is over, but that didn't abolish all the evil in the world. Voldemort is still rising in power, and although he isn't nearly as terrible as Mother Earth, he is dangerous. And with Percy by his side, there is a lot the man can do. But will he succeed in manipulating the greatest hero to ever live, or will his own grandson be his bane?

This is different from my previous Percy Jackson and Harry Potter crossover, for it dives into a new realm of loyalties and adventures.
It's not at all related to the former version.
And hopefully, it's also better written. It has been two years, after all!