I'm guessing since this is a post S3 fic you meant 'former' but having Moriarty referred to as Sherlock's 'formal enemy' did make me laugh.
I'm not sure which image I like better Sherlock and Moriarty fighting each other in tuxes or sending engraved formal invitations to become each other's nemesis.
Thereby prompting honorh to compose Moriarty's formal proposal of arch-enmity:
My Dear Sherlock Holmes,
I have admired your work from afar for many years, having not had the occasion to make your acquaintance. Upon my oath, however, I find I can no longer wait for a formal introduction. I must throw all caution to the winds and speak from my heart. Would you do me the honour of being my archnemesis?
Very Truly Yours,
Context dons its best Sunday Fighting Trousers.