Sometimes, Sherlock likes to imagine that he's a dragon.
...Well, only when he was younger, of course.
Criminals would tremble at the very hint of his name; Smaug, as he decided to name himself... He means the dragon, of course.
He would be an exceedingly large, fire breathing beast. Hailing from the Withering Heights, his wrath would be unstoppable even as he's a young drake.
Erebor would fall easily under his spear-like claws - it was a place he made up, in one of the quieter corners of his mind palace. This room was specifically labeled 'Middle-Earth'. He had designed a whole world to his liking. He used to retreat there much more in his earlier life, when the outside world was much too dull for his tastes.
As his natural life progressed, he became closer with a human named John. Internally, he labels him as a Hobbit. He renamed him Bilbo Baggins, as well. Middle Earth became used even less with his...friend around.
But it was not forgotten.
He fell - off of Saint Bart's to be exact - but his previous life was not the only thing that had crumbled. He was forced to jump by Moriarty - Bard - and within the deep ravines of his not-yet extinguished story, a Black Arrow pierced his thick hide.
His one weakness had been revealed, and he was now paying the price.
Sherlock traverses the night, slowly hunting down every last person that had contributed to his fall (he wants them to know that they will suffer. That their fate will be horrendous and painful by his own hands). He begins to see himself in a darker light.
He makes a new addition to Middle-Earth. He calls it the Necromancer. He is seemingly torn over which he views himself as - the great and previously unstoppable Smaug the Magnificent, or the dark and evil Necromancer that will coat the land in eternal darkness.
He can't decide.
Thinking of John's point of view, he would be the Necromancer - he covered his best friend's world in darkness. (He'll wait for me. He has to.)
But he knows that he will return. That he is strong, and Magnificent. Sherlock believes that he will be able to turn his weakness into his strength - he has to. Just as Bilbo stole the Arkenstone from his hoard, John stole his friendship.
So he settles for identifying with both.