Found! Shadow creature!Castiel/Dean
I’m looking for a very old fic where Dean is a kid and he meets this shadow creature Castiel. Dean carries and “births” Castiel’s children (I guess you could call this Mpreg) which are more tiny shadow babies. At some point they have a lot of kids together and Dean has named them. Eventually Dean gets caught and John and/or Bobby tries to destroy Dean’s babies. Dean disappears into Castiel’s shadow portal to protect Dean and the children. I think Dean later finds out that Castiel might be a fallen angel or something. I can’t remember if Castiel and Dean had a sexual relationship and I can’t remember where I read this fic.