"Tortoiseshell" is a Grandmaster of Demonic Cultivation: Mo Dao Zu Shi fanfiction by nirejseki focusing on the Nie Brothers. It was written in response to the prompt that Nie Huaisang has oracular abilities with a very steep price...and an older brother who doesn't know what to do.
It can be read here
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"Tortoiseshell" contains the following tropes
- Ambiguous Ending: It's left unclear whether Nie Mingjue's plan to avert the Sunshot Campaign himself, rather than cause Huaisang to do it, will keep his brother safe.
- Big Brother Instinct: Nie Mingjue is very protective of his younger brother and hates that he's suffering from his oracular abilities. By the end, he's determined to assassinate another sect leader himself to keep Huaisang safe.
- Blessed with Suck: Oracles in general are this. Sure, they can see the future, but if they change it, they feel any pain that would've been felt by others in the future. Nie Huaisang suffers basically chronic pain and most oracles go mad.
- Deliberate Injury Gambit: Nie Mingjue broke his own wrist to keep his father from going to see Wen Ruohan and getting his saber shattered. It works
- Dreaming of Things to Come: How oracular abilities work. Oracles see the future as it is without any changes.
- I'm Having Soul Pains: During the fic, Huaisang is suffering from "the unutterable agony of a saber breaking" as a result of averting his father's visit to Qishan where Wen Ruohan would've broken his saber. It's not made clear specifically how this is articulated, but it's clearly very painful.
- Loophole Abuse: Nie Mingjue's line of reasoning about personally averting the Sunshot Campaign without Huaisang telling anyone else about it: if he does it instead of his brother, then Huaisang won't have changed the future and avoids any pain that might come from it. The fic leaves it ambiguous as to whether he's right.
- Mad Oracle: Not Huaisang, but apparently a very common fate of oracles is to go mad from the pain.
- Straight for the Commander: Nie Mingjue's plan to avert the Sunshot Campaign? Kill Wen Ruohan, his (probably correct) reasoning being that he was the one who would cause it in the future.
- Waif Prophet: Nie Huaisang, who's a very young child. It's not made clear how young, but young enough that all it took for Nie Mingjue to experiment with his gifts earlier on was to bribe him with candy.
