The fic as I remember it had Sam being rescued and there were markings on his arms plus a necklace. Whoever had the necklace was able to command Sam to do what they wanted. I remember at some point Sam had lost the necklace and Dean found it. He put it in his pocket and started telling Sam to do things - on accident but then Dean started to have fun with it. I remember near the end of the fic Dean got angry with Sam and commanded him to sit in a chair with his eyes closed until Dean came back. I believe Cass was the one to find Sam and he made Dean apologize to Sam when Dean finally came back. As a punishment, I believe Cass turned the spell/curse on Dean but didn't actually use it against Dean. I hope that's enough info!