Soulmate Au but it's Johnlock!
It's the Au that if you write something on your skin with pen or marker, it appears on your soulmate's arm and vice versa. But, writer's plot twist! That's just Johnlock. Mystrade and others have different aus that vary. Johnlock is the most undetectable one, so Mycroft always assumed Sherlock didn't have a pairing for him. All of John's peers bullied him for not having an Au. But, growing more in age and somewhat in maturity, others began writing on their skin and both of them discover that they're alone. Not anymore.