John is a magical drain hole

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I am searching for a Sherlock magical AU, John is some kind of magical drain hole, he can suck magic in, when he touches magical objects they become ordinary, wards around him fail etc. This of course makes him rather dangerous in a society where magic is used everywhere which is why he keeps it a secret and tries his best to minimize the damage he does.

Moriarty is a very powerful sorcerer in this. I especially remember this one scene at the swimming pool where Moriarty reveals his grand master plan and Sherlock is just completely desperate because he can't see a way out. John on the other hand is just amazed that Sherlock hasn't figured out what he is and drains all of Moriarty's wards and spells. Anyone know this?