It happened during summer break. Mycroft was about 12 years old at the time, and was considered to be old enough ,or at least by their nanny, to watch a then 4 year old Sherlock while she went off to buy some milk. In the fifteen minutes she was gone everything procceeded to go to hell, and despite the fact that it wasn't really Mycroft's fault, he was never trusted with watching his baby brother ever again.
It had started easily and simply enough, Mycroft had waited about five minutes to make sure the Nanny wasn't just testing him, and was, in fact, really gone. At which point he proceeded to pick the lock on their father's library/study so that he could gain access to the "grown up" books that he and Sherlock weren't allowed to read. Mycroft had left Sherlock outside by himself, playing in the gated garden, where he was not to leave under any circumstances on pain of many future wedgies and pranks while he raided the study of any books that looked interesting to him. Though this in itself might seem like a reckless thing to do, it is important to note that the garden was fenced in, and the gate (at least at the time) was locked, as well as that Sherlock was much much more mature than most four year olds...
Anyway, Mycroft was in the study, looking for books he found interesting. Most of them were books that were too old or expensive for Mummy or Daddy to be comfortable to knowingly let him read not under their supervision, such as old books of fairy tales, or a first edition copy of the hobbit he kept revisiting. Others were things that they felt the boys shouldn't be exposed to until they were both old enough that they could understand it without getting nightmares, such as an old medical journal, and several horror stories. The most uncommon and also Mycroft's favorites, were the books that fell into both categories.
One of his favorites of these books was an old well worn book on the supernatural which had been passed down along the Holmes line for generations. Although Mycroft knew that it was all hogwash, and likely only amoung his parents' collection of things because it was an heirloom, he couldn't help but be facsinated by the macabre , sinister, and at times, even gory, book that so many of his ancestors had gone through that it was permanently dog eared. He also loved reading the book, because (like most children that age) he was facsinated with paranormal, and the idea that a Holmes man(or woman) had written the notes in the margins of almost every page on different ways to appearently kill various creatures made him feel special; Almost like a superhero.
It was this book that Mycroft was reading by the then empty fireplace in the living room when he heard Sherlock scream.
Now Myrcoft knew that Sherlock had a tendency to overreact to certain things, a simple bug, usually a bee specifically, was something that could send him into a hysterical crying and screeching fit;However, something about the tone of the scream seemed... Off, and seemed to be less about fear, and more about pain. It hit Mycroft that something was wrong so quickly that he had already gotten a fireplace poker on his way to the garden, where he saw something extraordinary.
A huge, titanic, hulk of a beast that could only pass for a dog in the barest sense of the word, had somehow gotten past the thick protective fencing around the garden and through the gate, and was now chasing Sherlock around the garden,making a noise halfway between a bay and a growl as it did so.
By the time he got within strikeing distance, it had gotten Sherlock by his shirt and was already clawing and tearing and the boy.
Luckily for Mycroft, the thing seemed so surprised to have someone actually hit it instead of running away screaming that it paused, and looked at him with a puzzled look of confusion.
This, of course, gave Mycroft the perfect opportunity to hit it again.
And again.
And again.
And again.
However, after the fourth time it was hit over the head with an iron fireplace poker, the creature seemed to regain it's wits, and became quite...upset that someone was beating it about the head and neck with it, and started to actually growl at Mycroft, causing Sherlock, who was luckly in decent enough shape to hide behind his older brother after the creature had let go of his shirt with the first blow, to whimper.
The beast then charged at Mycroft, who held the poker straight in front of him like a spear, and stabbed it straight throught the chest. He then proceeded to pull the poker out of the beast and stab it again and again and again, until long after it had stopped shivering and whimpering, and he was covered in it's black blood, and Sherlock had to tug on his shirt to bring him back to reality.
Then the beast dissolved into a pile of black sludge.
Sherlock, who was now bleeding much more than a 4 year old should from his wounds, spoke up..
"Mycroft, what was that?"
"It was just a dog, Sherlock"
"No it-"
Mycroft turned and gave Sherlock a wild look that shut him up immediately.
"Sherlock, it was just a wild dog, ok? You were just imagining things."
This made Sherlock cry., which in turn finally made mycroft realize how serious his wounds were.
When the Nanny got home, she saw that emergency services were swarmed all over the property, as well as the cars of Mr. Holmes (who'd been the second numbers that Mycroft had called after 999) Everyone had noticed the fact that Mycroft appeared to be covered in what looked like oil, but what was obviously the blood of the dog that had attacked Sherlock, as well as the fact that he had stood guard over his little brother while they were waiting for the ambulances in the kitchen. However, no one seemed to notice the salt circle that Mycroft had made around his little brother, or the look of fear and understanding in Mycroft's eyes, or the centuries old book on the table that was open to a page about hellhounds