The Turn of Haruhi Suzumiya

“I was about to be thrown in to a living nightmare, one far more terrifying and dangerous than anything I’d faced before. And this time, Haruhi was at my side through the thick of it all. The one person who isn’t supposed to know that the supernatural exists.
I thought traveling through time, dealing with alternate realities, the mansion in the snow, kidnappings, car chases, weird divesting realities and the Anti-SOS Brigade had left me ready to deal with anything that the universe could throw at me.
I was wrong.”

The Turn of Haruhi Suzumiya is a Haruhi Suzumiya/Umineko: When They Cry crossover fanfic written by Oroboro. It combines Kyon’s snarky and focused narration in the setting of murder and mystery of Umineko.

A basic summary: For the second summer vacation trip, the SOS Brigade visits the island of Rokkenjima. Expecting a simple trip of camping, hiking, and pretend mystery solving on a long deserted island, Kyon and Haruhi instead find themselves pulled into the game board, intruding into the Ushiromiya family conference of October 4th, 1986. Knowing full well the ultimate fate about to fall upon the island’s inhabitants, Kyon must race to solve the mystery and find the truth; before they end up as just another mysterious death in Rokkenjima’s endless cat-box.

It can be found here. Part 1 is 100% complete, and part 2; “Counterattack of Haruhi Suzumiya” has its prologue up here.

Tropes used in The Turn of Haruhi Suzumiya include:

Agent Mulder and Agent Scully: Kyon and Haruhi have always had this kind of dynamic, but it’s magnified here when Kyon easily accepts the magic going on and Haruhi rejects it entirely.
Anyone Can Die
Arson, Murder, and Jaywalking: Discussed by Kyon when he mentions Jaywalking is as far as he’ll stretch his morals in a situation like this.
Bloody Murder
Bolivian Army Ending: Kyon and Battler face off against an army of goats. They don’t make it
Boom! Headshot!
Break the Cutie: Battler starts off pretty broken, and Haruhi breaks down over the course of the story, culminating in the Tea Party.
Brick Joke: Haruhi snores.
Brilliant but Lazy: Kyon somehow manages to combines both traits and uses them to defeat Belphegor. Of course, Nagato was helping too.
Brown Note The Red Truth can’t be understood by normal humans on the game board, and leaves Haruhi utterly baffled when she somehow uses it.
Can You Hear Me Now?: Hard to get cell phone service on a deserted island in 1986. At least a useless phone doubles as a flashlight in a pinch.
Clarke's Third Law: Kyon doesn’t really care how Nagato’s, or anybody else’s powers are explained. It’s all the same as magic to a normal person.
Darker and Edgier: Darker than Haruhi, but about on par with Umineko.
Does This Remind You of Anything?: Several scenes are intentionally reminiscent of scenes from the source material.
Drowning My Sorrows Battler has given up and fallen into this.
Flat Character: Most Umineko characters don’t get a lot of time spared for characterization. But, Kyon’s only a single narrator and there are an awful lot of them.
Flat What
For Want of a Nail: Kyon refers to this one by name, comparing himself to Battler.
Improvised Weapon: Kyon's trusty curtain rod, a solid but hollow tube of brass.
It Got Worse: Kill'Em All When the seagulls cry, there are no survivors.
Lampshade Hanging: Everyone’s favorite narrator refuses to leave a bulb uncovered as he’s confronted with the world of Umineko.
Leaning on the Fourth Wall: Kyon spends a lot of time doing this, especially once he figures out where he’s ended up. Coupled with the occasional This Is Reality
Maybe Magic, Maybe Mundane: It’s the whole purpose of Umineko after all.
Mood Whiplash: Kyon's narration can swap between snark and horror at the drop of a hat. Invoked in-universe when he's elated after beating Belphegor and then his mood immediately falls when he turns around to remember Jessica's corpse
No-Holds-Barred Beatdown: Kyon gets the crap beat out of him by Eva.
Pretty Little Headshots
Shoot the Shaggy Dog
Shout-Out: More than some, less than others.
Troll: Bernkastel is revealed in the “????” chapter to be up to her usual antics.
Wham! Episode: Some of the chapters, but the Tea Party probably hits the hardest.
What Happened to the Mouse??: Done deliberately: Kyon completely forgets about the message he finds in his bathroom. Twice.
What You Are in the Dark: Battler is implied to have faced this and failed.