I have been trying to do a Nasuverse story pairing Harry with Hassan of Serenity for a while (amongst many other story and pairing ideas), but until I played through the Camelot Singularity of Fate/Grand Order, I couldn't find the right 'in' for the story. That particular part of the game has one of the better stories in the game (no surprise, given that Kinoku Nasu himself wrote it), but also the most notoriously unforgiving battles in the game, as I discovered firsthand, despite being forewarned.
Now, this story is set between the end of the Epic of Remnant arc and the beginning of the Cosmos in the Lostbelt arc. Two years have passed between the Camelot Singularity and this one, with much of that time spent dealing with other singularities, and Harry's age has been advanced a year, so he was nearly 16 when he ended up in the Camelot Singularity.
Also, I would like to give my thanks to GhostXavier, the writer of the excellent Fate/Grand Order slice-of-life fics Fragments of Chaldea and The Lost Fragments, as well as fanfics revolving around the last three Singularities of the game's first story arc. Not only has their story helped shape my own works, and not just this one, but they have also kindly given me permission to use their OCs in this Fate/Grand Order fic. And while GhostXavier's OCs will not be having a prominent role, at least in the chapters written so far, you may see more of them as time goes on.
Anyway, time for the usual disclaimers. Firstly, there will be spoilers for both Harry Potter and Fate/Grand Order.
Secondly, there will be heavy annotations, as is usual for my works. You have been warned.
Thirdly, this is an M-Rated work. There will be coarse language, violence, and sexual references. Again, you have been warned.
Finally, the following is a fan-written work. Harry Potter and Fate/Grand Order are the properties of their respective owners. Please support the official release. Otherwise, Fou will bite you on the arse…