Fate/kaleid liner Prisma☆Illya Movie: Sekka no Chikai gives us a glimpse as well as a proper detail towards the actual alternate world Miyu was born in through the lens of the alternate Emiya from 3rei!.
Now having alternate Emiya as our main narrative focus maybe an odd choice since this is suppose to be you know, Miyu's backstory and how she got into Illya's world and the such, but nonetheless alternate Emiya himself ALSO has a story of his that fills in the details and is obviously linked to Miyu and everything that happened in 3rei! as well for things to to go "full circle" as it were.
With the main narrative clutch of Emiya and by extension Kiritsugu wanting peace any way possible even for the life of one in order to do it and using Miyu as the vessel as the sacrifice brings attention and questions Kiritsugu's ideals in the first place, with Emiya's beliefs towards it as he gradually loses everything dear to him starting from Miyu taken away by the Ainsworths and Emiya pretty much in this alternate world anyways, turning into a proto-Kiritsugu in his Fate/Zero days. On the other side, Julian Ainsworth actually wants the same thing but his methods and care for Miyu is not even close to humane and he actually embodies Kiritsugu's ideals of wanting to peace for humanity but he'll WILL DO ANYTHING TO DO IT even at the cost of one's life. Fate/kaleid liner Prisma☆Illya Movie: Sekka no Chikai's thematically questions whether Kiritsugu's ideals not only in this movie but heck Fate/Zero was the right thing to do, kill one in order to save everyone and both made manifest with Emiya still believing in that but more like save all with no deaths, while Julian will kill for humanity.
Due to Emiya taking up the majority of the spotlight as well as his development into proto-Kiritsugu, Miyu doesn't get that much screentime, but what we got at least gives us what Miyu's personality later on in Fate/kaleid liner Prisma☆Illya prosper into.
Fate/kaleid liner Prisma☆Illya Movie: Sekka no Chikai also ties up loose ends towards 3rei! and why the Holy Grail War finished already and that was because of Emiya and by the ending itself how Emiya later ended up in 3rei!.
On the animation side, done once again Silver Link, seems aesthetically if not BETTER especially during the fights towards Fate/kaleid liner Prisma☆Illya series. Same character designs as it's original seasons and beautiful backgrounds and awesome fights rivaling ufotable's works and Fate/kaleid liner Prisma☆Illya Movie: Sekka no Chikai much like 3rei! doesn't skimp out on the gore and blood either. What can LA say, LA was VERY impressed with the animation done by Silver Link to be almost on par with ufotable's Unlimited Budget Works grade of animation.
Voice acting-wise, since this movie is set in alternate world but in the Fate/stay night and some from 3rei! as well, ALOT of the voice actors reprise their roles. Noriaki Sugiyama gets most of the screentime as Emiya and that's a given but nonetheless he did a great job, Kaori Nazuka was good for the few moments Miyu is on screen though most of the time she hardly spoke but LA can give some grace with that, as she showed a bit of emotion here and there. Some other standouts are Natsuki Hanae as the psychotic yet calculating asshole Julian Ainsworth and the actual psychotic asshole Shinji Matou as Hiroshi Kamiya for their performance. Great cast what else can LA say?
Fate/kaleid liner Prisma☆Illya Movie: Sekka no Chikai is without a doubt a movie that needs to be watched by the fans of the Fate/kaleid liner Prisma☆Illya series especially if you want an in-depth perspective from Miyu before the beginning of the anime series, sure Emiya has most of the screentime but Emiya still fits anyways with how strong it's thematic cohesion of "kill one to save everyone" is put into question with Miyu at the center of it all and better yet to link to how this movie kickstarted Fate/kaleid liner Prisma☆Illya as a whole and tying loose ends in both Emiya and Miyu's end from was great in all honesty.
"Would you kill one to save everyone?"
