You see, the 'High-Priestess-can-erase-memories-thing' would be extra creepy if even the vampires didn't know about it. It was this big huge secret that the High Priestesses (are there any High Priests?!) kept from everyone for whatever reason. So, when Marx's sister found out by accident (for fun, I'm going to say a friend of hers had some memories erased that she knew about so she investigated it and thus discovered the secret), she knew there would be absolutely no way anyone, vampire or human, would believe her, so she told her beloved brother.

It would give a level of creepiness, make High Priestesses have this weird, ultimate control over fledglings and vampires (and thus, PLOT!), and explain away why vampires didn't share it with humans: can't share something you don't know about! On could even pull out that this is the reason humans have such a distrust of humans: there are rumors there are vampires that can erase enemies, the humans found out and demanded the vampires came clean, yet, the vampires legitimately didn't know what they were talking about. So, mixed with old vampire legends and this new huge rumors, humans and vampires don't get along.

That took me about a minute and I extrapolated from one sentence. I have everything the Casts want.

Big vampire secret that humans don't know about? Check.

High Priestesses are evil? Check.

Vampires look misunderstood? Check.

Humans jumping to conclusions? Check.

A plot that can span multiple books? Check.

Me being awesome and showcasing how I'm 100x more creative than both Casts put together? Triple Check.

If I knew it would be this easy to write a mediocre vampire book, I would have traded in my dragons.