In response to this prompt, which I found on AO3: A Young!Sherlock fic. Might even be a kid fic, though a more 'serious' story is preferred - something from Sherlock's past when he still lived at home with Mummy and Mycroft, maybe even his father. Pretty much any kind of backstory.
Age 8
Mycroft comes to my door. Knocks. Knocks? Something's wrong. Not a holiday, I ate last night. Not himself, never asks me for help. Mummy. Launch myself off the bed, run to the door. Open it.
The bruises are all I need to see. Rush to Mummy's room. Mycroft doesn't seem to mind. Father's nowhere in sight.
Good.
I hope he's never in my sight again.
Go to Mummy. She's on the ground, fetal position. Too familiar. Don't touch her; she'll shy away. Speak softly.
"What happened?"
Glances up. Hair falls in front of her face. Deliberate; doesn't want me to see. Brush it aside anyway. Face covered in bruises and cuts. Sharp cheekbones, one of them bleeding freely. Lost more weight, too gaunt, too pale. Eyes wide. Bruise on her forehead, slightly bloody. Not caused by a hand, likely a club or other blunt instrument. Glance around; lamp is lying on the ground. Turn attention back to Mummy. She's regained her composure, is sitting up now. Wipes her eyes calmly; mouth tightens at the edges. Trying not to wince. Other injuries, probably kicked her. Flash of anger.
She sees.
"Don't let your emotions get the best of you, Sherlock. Especially not now." Her voice is strong, doesn't shake.
I nod. I cannot control my father, but I can control my mind.