Ritual Habitual by dazzlebug. I'm also stealing the text of her rec:
Seriously, that's all I can say -- this video is SO FREAKING AWESOME. It's a Supernatural constructed reality, and it's basically flawless It's twisted and dark and violent and so well done, it gave me chills and had me gasping at the screen and I couldn't love it more IF I TRIED.
Watch it. SRSLY. *flails around*
Yeah, seconded. I haven't been that creeped out since War Drums. Beautifully done.