story description:  
Shawn Spencer, psychic detective, is a thick-tufted boy genius who ice skates through life on polished blades of snarky eloquence. He has tested himself against some of the greatest criminal minds of his time and emerged victorious… though often through unbelievable luck and not a small amount of bumbling.
 
Now, he faces a challenge unlike any before. One that will test his mind, his sanity, and
 
…  
his adaptability as he is thrust into the midst of an unfamiliar world and the ponies who populate it.
 
The Mane 6 face their own challenge, as an old enemy, tempered in the crimes of a much darker world than theirs, returns to Equestria. It is a challenge they are ill-prepared for and one that only their strange, new, alien friend can help them overcome.  
link to the fic is [url=http://www.fimfiction.net/story/4631/Magic~2C-and-Mayhem~2C-and…-Murder%3F-Oh-my…\]here\[/url\]