Ianto merges with/is turned into a blob alien
Read this more recently, idiotically forgot to bookmark it. It's an older fic though.
The team are tracking an alien, they're invertebrate blobs of some kind. One of them latches onto Ianto and when he's severely injured psychically convinces him to let them merge together in order both save him and to help defeat the other aliens, which are like a similar species but evil (or something like that). At one point they're in the sewers and Ianto has mostly merged with the alien, unbeknownst to the rest of the team. They point a gun at him but he's still Ianto enough to be able to talk and convince them to let him live. He ends up living in a cell and at some point he and Jack have funky tentacle sex. I think it ends with the alien part deciding to die in order to let Ianto live again as a human.
The team are tracking an alien, they're invertebrate blobs of some kind. One of them latches onto Ianto and when he's severely injured psychically convinces him to let them merge together in order both save him and to help defeat the other aliens, which are like a similar species but evil (or something like that). At one point they're in the sewers and Ianto has mostly merged with the alien, unbeknownst to the rest of the team. They point a gun at him but he's still Ianto enough to be able to talk and convince them to let him live. He ends up living in a cell and at some point he and Jack have funky tentacle sex. I think it ends with the alien part deciding to die in order to let Ianto live again as a human.