I've lost track of a really awesome fic I read and have had no luck finding it again. I think it was written post 9x23 and it details Demon!Dean going on a rampage/killing spree through Vegas and the ensuing police chase. Sam and Cas, of course, are hot on his trail and eventually find him and cure him. The damage is done, however, and Dean is consumed by guilt over what he's done. He disappears from the bunker (I think) and starts criss-crossing the United States. He sends Sam and Cas back post cards and souvenirs from the towns he's visiting...but some of the post cards have numbers written on the back. The numbers decrease the longer Dean keeps sending them. Eventually Sam figures out that Dean is visiting the sites of past disasters, natural or otherwise, and writing down the death tolls. When Dean finally reaches Vegas again, the site of the disaster he caused, he sends a post card with something like 'Vegas massacre, death toll 40-something, the 200th(or close) worst disaster in US history'. Then the post cards stop. Dean eventually finds his way back to Sam and Cas, but my recollection of that part is hazier. I hope someone else has read this fic and can direct me back to it. It's the kind well-crafted writing that should be read more than once.