"My name is Castiel, and I’m going to get you out of here. You might want to hold on to something."
Portal 2 & Supernatural Crossover AU, where Hell is the Aperture Science Enrichment Centre.
After years of life-threatening tests in the name of science, thanks to a malicious AI named AlisTR (Artificial Laboratory Intelligence System, Type: R), Dean Winchester wakes up to realise that the whole complex is falling apart. All he has is his wits, a portal gun, and the help of a robotic personality core calling itself Castiel to try and get out before he ends up dead, or worse.