When he was 5 years old, Sherlock's magic began to make itself known in the typical fashion.
The first incident occurred when Mycroft informed him, with a smirk on his face, that Father Christmas was not real. Sherlock, hoping to appear unaffected, stood clenching his fists and straining to hold back inevitable tears. As he glared at Mycroft through the precarious droplets on his lashes, he saw what appeared to be a raincloud forming above their heads.
In the years that followed, Sherlock enjoyed reminding his family that he had successfully stopped his tears from falling. Conveniently, the raincloud tended to go unmentioned.
It rained in their kitchen for thirteen days.
When Sherlock turned 11, a letter arrived in the post.
For the Holmes boys, magic had never been a guarantee. Although the gene was almost always dominant, they knew that Half-bloods ran the risk of presenting as Squibs, and Mr. and Mrs. Holmes made it a point to assure their sons that they would love them either way.
And so, when the day came, Sherlock felt free to make a choice.
For many years, magic had been nothing but a burden, bringing to life his most turbulent emotions. He found no beauty in it and felt no desire to let it bloom within him. He had grown to be a practical and studious young boy who rather preferred doing things the long way, whether he was conducting a science experiment or solving the latest mystery in his town. What fun would it be to use a spell when it was quite possible to use his own brain?
Sherlock sat on the edge of his bed for hours that night, turning the thick envelope over and over in his hands and thoughtfully weighing his options. As he pondered, the envelope slowly became crumpled and dirtied.
In the end, he never opened it.
The years went by, and Sherlock immersed himself in the Muggle world. He became quite good at hiding his magic and often forgot about it entirely. But in the absence of any formal training, his abilities never progressed, and he did not learn to control them as most wizards did. The energy that ran through him was not properly dealt with, and as he buried it deeper, he began to believe that his powers may have disappeared completely.
And then, a soldier walked into Barts.
His magic came to the surface almost immediately, causing a tingle in the tips of his fingers and a spark in his belly. Standing outside of 221B the next day, exchanging a fleeting handshake with John Watson, Sherlock felt the spark burst into a flame.
God help us, he thought, struggling to hold the feeling at bay.
Merlin help us, a small voice from the depths of his Mind Palace corrected. God doesn't even own a wand.