Percy Jackson goes to Hogwarts

What happens when someone knocks on the door to the home of Percy Jackson himself? What happens when Percy and his mother find out that he has another evil grandfather? Read this story to find out.
DISCLAIMERS: - Not a completely original idea. - I own that plot. Not the characters or settings, that stuff belongs to Rick Riordan and J.K. Rowling.
SETTING: - After the war with Gaia for Percy. - In the Order of the Phoenix for HP/Hogwarts.