Memories=Bye bye

It's been three years since Percy Jackson and Harry Potter met.
The Giant War was long over, and the Battle of Hogwarts was successful.
But when a new threat rises in the mythological world, the two camps aren't enough, and Percy has no choice but to call reinforcements from Hogwarts.
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Harry was nothing but confused when a strange man with a weird resemblance to himself showed up at his door.
Even weirder, he wanted Harry's help in another battle.