The fic that turned me into a SessKag fan was "A Demon's Heart and Ultimate Sacrifice," by Kilala81. It was a hauntingly beautiful story.
Aha, and "A Simple Legacy," by Elle6778 came out right about the time I started reading, That story - and its sequels - were AMAZING.
And they just kept coming! From the early days:
"Tears of the Fallen," by ShadowsWeaver1 is beautiful, I just re-read it.
"Trust," by Demonlordlover, which is unfortunately no longer on any of the sites, was a great story.
And everybody's perennial favorite, "Tales from the House of the Moon," by Resmiranda. I stayed up for days reading it, on like 3 hours of sleep and almost no food
These fics turned me into a SessKag addict.
Oh, and the fic that turned me into a SessKag hentai was "Passion," by CiraArana. I've read it a billion times, and it's still wonderful and still makes me blush. You have to make sure to read it on Spark, though, because all the good stuff is cut out on FF.net - it was written during that M-rating crackdown.