I'm looking for a specific fic. It's a Dean, Sam, and Cas fic of the lengthy, 'epic' variety. I believe it was Gen, but it might have been Dean/Castiel.

In the introductory scene, Castiel asks Dean and Sam to investigate a house to which he's been shielded against, and inside they find a lot of gore, dead demons, and an altar with a mutilated corpse being used as a conduit for something trying to crawl out of Hell. The corpse grabs at Dean, and it's so strong that Sam has to hack its arm off and the altercation leaves all shaken.

From there, I believe they attempt to investigate the phenomena. There might be several minor cases involved before finding a resolution to the initial problem. I don't remember much beyond the introduction, but I believe the driving plot motivator was the unease following the initial encounter.

Any help would be much appreciated.

Found: Last One Out Hit the Lights