Shut Hell

Shut Hell (also known as Shuto Heru) is a Seinen manga by Itoh Yu.

The story opens in present-day Japan, where a young man called Sudou is plagued with recurring vivid dreams of bloody ancient battles in which a young girl and her comrades defend a doomed fortress. While meeting friends, Sudou is approached by Susuki, an apparently shy and quiet schoolgirl who has sought him out for some reason. Sudou's family were musical instrument makers, and when Susuki visits his apartment she feels compelled to play an instrument he made. As she plays, Sudou is mystically transported to ancient China, where he awakes to find he is now a woman nicknamed Shut Hell, and Susuki is now Yurul, a young Mongol prince.

The story takes place in the early 13th century, in what is now North-western China. Genghis Khan has united the Mongol clans, and sent his forces to attack the territory of the Western Xia (called the Tangut Empire in the manga). Our heroine was a Tangut soldier nicknamed "Little Sparrow", sole survivor of a unit massacred by forces led by Mongol prince Harabal. Now driven by vengeance, she is feared by the Mongols as "the Evil One" for her merciless killing. She accompanies Yurul as he flees his Mongol family for Sou territory to preserve a vital collections of writings, knowing that Harabal will follow them, giving her a chance to kill him.

Tropes used in Shut Hell include:

Anti-Hero: Shut Hell, who is pretty morally ambiguous.
Action Girl: Shut Hell.
Ax Crazy: Shut Hell is prone to killing people very violently.
Back from the Dead: Sudou wakes up as Shut Hell swinging naked from a gallows. Yurul implies this isn't the first time she's come back.
Badass Bookworm: Yurul.
Badass Grandpa: Boldo.
Big Brother Complex: Harabal towards Yurul, all the way. Yurul doesn't seem to feel nearly as much towards him, though.
Big Ol' Eyebrows: Apparently the family trait of Harabal's family. His father has big ol' eyebrows, he has big ol' eyebrows, and Yurul... wait a sec. Hum, which one could possibly not be truly blood related to them?
Blood Knight: Shut Hell
Break the Cutie: What happens to Little Sparrow. The transformation is so great that it earned her the nickname of "Shut Hell" (Evil One). Yikes.
Cool Old Guy: Boldo.
Cute Shotaro Boy: Yurul can be quite cute.
Doomed Hometown: It isn't probably isn't actually the heroine's hometown (we don't really know), but all the same it's got the other makings of the trope: Her friends are killed there, she seems to be the only survivor, and it does start off her epic journey.
Dream Sequence: It starts off with a Dream Sequence that feels more like a Flash Back.
Everybody's Dead, Dave: Little Sparrow's regiment, leading to her huge Break the Cutie moment.
Flash Back: The Dream Sequence. Also most of the story so far is told through an extended flashback.
Gender Bender: Not that it really invokes any of the genre's tropes, just that the two characters' genders are the opposite of what they used to be.
Heroic Sociopath: Shut Hell.
Historical Fiction
Hot Amazon: Shut Hell.
In Love with Your Carnage: Alfald towards Shut Hell. He's shown to get crazy excited when he sees her killing. "Yes, show me even more, Shut Hell. More than gold, more than women, fascinate me more. With this wind of violence only you can show me. Take away my soul!"
Loners Are Freaks: Yurul. Lampshaded by Harabal, who tells him he's "always alone," and that his music is always sad, so it always drives people away. Yurul responds that he's "no good with people," and the things he values doesn't make sense to other people.
Loyal Animal Companion: Yurul's pet eagle Yaralt.
Morality Pet: Yurul for Harabal. Harabal's father, the clan leader, notes that Yurul is one of the few things Harabal holds precious and dear.
Mummies At the Dinner Table: Shut Hell keeping all of her friend's corpses and talking with them. Not to mention how rabidly she defended them against hungry wolves.
Naked on Arrival: When Sudou goes back into past into the body of Shut Hell, s/he's... very naked.
Old Retainer: Boldo towards Yurul.
Parental Abandonment: Sudou's parents walked out when their business failed.
Perma-Stubble: Harabal.
Pet the Dog: What Harabal does with Yurul. He even literally pets him.
Playing the Heart Strings: One of the warriors hates Yurul's music because it makes him want to "blunt his fangs" and stop fighting.
The Quiet One: Yurul.
Rebellious Prince: Yurul.
Reincarnation
Roaring Rampage of Revenge: Shut Hell is hellbent on killing the "Tiger" (Harabal) and any of his accomplices, as revenge for killing all her best friends.
Royal Blood: Yurul is secretly the Khan's son.
The Runaway: Yurul becomes this of his own accord (taking along Old Retainer Boldo).
Scars Are Forever: Harabal has quite a few scars on his face that are permanent.
Speaks Fluent Animal: Shut Hell might be able to communicate with wolves, though it's not clear if it's all wolves, or just the one (suggested to be a Mongol god), or if they're both special, or even that it wasn't a hallucination.
The Stoic: Yurul can be considered this. Most of the time, his face doesn't show nearly as much emotion as the other characters.
Time Travel: What happens in the beginning with Sudou and Suzuki.