No, not Nanoha's mortal enemy and future girlfriend Fate Testarossa. It's a franchise crossover.
I'd been looking forward to this. "Fate/kaleid liner PRISMA ILLYA" is fantastic. "Magical Girl Lyrical Nanoha" has serious writer-related problems, but that needn't drag down a quick comedy crossover. Maybe they'd even take the mickey out of it?
Besides, they're a good fit. They're both magical girl franchises with ten-year-old heroines that are in fact spin-offs of well-known pornographic computer games. Despite still being aimed at a male target audience, both conform to their fluffy new genre. Well, apart from apocalypse-level magical battles, lesbians, etc. Both also have characters who could talk German to each other.
Then I saw that the manga's written by Masaki Tsuzuki. My optimism took a hit, but I kept reading.
CHAPTER 1 (4 pages): "Fate/stay night x Magical Girl Lyrical Nanoha"
Fate/stay night is the Fate franchise I'm less familiar with, but that hardly matters for something this brief and incoherent. You'll be wondering if there's a page missing. However it's quite funny to see Nanoha and Fate get magically summoned, all huge-eyed and moe, looking about three years old... and of course being basically Gundams in dresses.
I enjoyed it, but it's almost worthless except for the painted art.
CHAPTER 2 (31 pages): "Magical Girl Lyrical Nanoha x Fate/kaleid liner PRISMA ILLYA"
Slightly less insubstantial. That's about the right degree of praise, I think.
It has a fair amount of comedy, thanks to Illya and Miyu. They're more entertaining characters than Nanoha and Fate. It's funny to see Illya and Miyu jump into a swimming pool with no water in it, although it also looks extraordinarily painful. (Someone I knew at primary school did that in his gap year before university and is now paralysed from the neck down. True story.)
Anyway, they're reality-hopping. Everyone's been sucked into an extradimensional space that's filling up with famous landmarks and getting ever more crowded. Illya and Miyu fight a mud monster and are in some trouble when Nanoha happens. Holy flaming cow. "I'm sorry, I couldn't adjust the power as I intended. Are you sure you're OK?"
So far, so good. Unfortunately this is Masaki Tsuzuki, so the rest of the plot is just the girls teaming up to shoot inanimate objects. He thinks this counts as a story. Lots of energy blasts and property damage later, we've almost used up the allocated page count! There's a comedy reveal about who's behind all this trouble, which made me laugh but does depend on having seen "Fate/kaleid liner PRISMA ILLYA". Everyone then goes home.
In fairness, Tsuzuki does actually get the comedy. Illya and her friends are funny. That's important. He also keeps up the franchise's perversion level, with Illya's fetish switch going into overload on seeing both Nanoha and Fate. That's despite the story having outed her only a few pages earlier as apparently incestuous-but-heterosexual. (Miyu's the lesbian, obviously.)
"Illya = a girl tricked by Magical Ruby into becoming a magical girl. Infatuated with her older brother."
"Miyu = current master of Sapphire and friend of Illya's. As a result of previous events has complicated feelings for Illya."
The narration also has a low opinion of Ruby, which seems reasonable:
"Magical Ruby = magical staff of love (?) and justice (?) that makes Illya into a magical girl. Source of all the turmoil."
Nanoha as always is a bit of a straight man (inappropriate phrase), despite a big close-up when talking about Fate and she says, "No... but... I think it would be great if she became my friend some day." I don't think this means anything age-inappropriate. Presumably it just means that these are the early days when Fate's still trying to kill her. However there is a mildly chilling bit where they all vow to kill the unalike. "Strike with Magical Girl Alliance! Our objective is to find a missing girl and to exterminate all those 'things' out there." Then, at the end, there's an unintentionally amusing bit when Nanoha says, "I surely... I surely think... that we found two new friends here!" Ah, two more friends she'll immediately forget about and never see again as long as she lives!
It's fluff, for the 5th anniversary of Comp Ace magazine. It's below the level you'd expect even from this kind of gimmicky crossover, but to its credit it has a few laughs. It has an appropriate level of inappropriateness. (Spot the boob grab!) It's above-average for Nanoha, but below-average for Illya and Miyu. That sounds about right to me.