Fic rec: Supernatural
It's been awhile since I've done this, but I just finished reading this and wanted to rec it. It's a nice long 54,500 words.
rei_c's Knowledge of Dead Secrets is an older fic. It takes Sam's S2 powers and twists them a new and interesting direction. Sam is a horse for three Voodoo Loa. It's essentially possession, but as with the angels, a Loa, who are intermediaries between the Creator and Humanity, must have permission to ride a human. Sam's character is very dark here. Even though it's set right before the pilot, this is not that Sam. He's not quite soulless!Sam, but he's dangerous and even ruthless at times. Dean doesn't know how to deal with this version of his brother that's nothing like the kid who left him and John to go to Stanford. Be warned, there is Wincest, but it's worked into the plot in such a way that it's not forced or out of the blue.
The author has quite a few works, and I'm looking forward to delving into more of them.