OH NO WAIT I found it. But it's actually from 2009, so maybe you've read it.
Dice, in a Game of War Summary: Fantasy-detective fusion AU wherein Waya, a private investigator in the city of Ki, is hired by Touya Akira to track down the previously exiled Sai. A series of mysterious magical deaths coincides with Waya's new case, and he finds himself having to face his own path - as well as find out the truth before someone else gets hurt in the process. https://archiveofourown.org/works/131801/chapters/187860
Re: You said you like long fics, here are long fics
Dice, in a Game of War
Summary: Fantasy-detective fusion AU wherein Waya, a private investigator in the city of Ki, is hired by Touya Akira to track down the previously exiled Sai. A series of mysterious magical deaths coincides with Waya's new case, and he finds himself having to face his own path - as well as find out the truth before someone else gets hurt in the process.
https://archiveofourown.org/works/131801/chapters/187860