For the sake of the fic, the Sherlock 'verse is a bit altered and turned into a universe where people have daemons (something apparently borrowed from Philip Pullman's His Dark Materials). I am not into AU and fantasy but, in this case, it sort of makes sense...
I read it because antennapedia , who is a good fanfic writer, recommended it; because it was Mycroft-centred and I find the character utter fascinating; and because the pitch of the first part was:
"In the pocket of his trousers, next to his phone, Mycroft carries the standard-issue steel capsule meant for protecting arthropod daemons. When people ask about it, he smiles and tells them she's not fond of the light, or of people other than himself. This is a lie."
Anyway, if you're curious, it's here.