The world quest titled “The Replacement’s Secret” is the second quest under the quest series “The Very Special Fortune Slip.” It becomes available to you after you complete the Shuumatsuban Operations world quest.
The last quest ended with Ms. Momoyo handing you a special bamboo slip that was scattered with some Shuumatsuban-made elemental powder. In this new quest, the first thing you need to do is go to the Grand Narukami Shrine, draw a fortune slip, and switch that bamboo slip with the one that Ms. Momoyo gave you.
When talking to the fake Gendou Ringo, you can also choose to just give her the bamboo slip without taking a fortune slip first. It may seem like she noticed that you did not take a fortune slip, but you can tell her you need it and that she was just too distracted to notice.
After that, you need to go to the hill by the shrine’s entrance and wait for the impostor Gendou Ringo to leave in the evening. You can time travel by adjusting the in-game clock to sometime between 21:00 and 23:00.
The impostor, whose actual name is Lyudochka, will go down the mountain, but she will move quickly and not give you a chance to see where she is going. However, since you gave her the bamboo slip scattered with some special elemental powder, you can use your Elemental Sight to see traces of the powder.
The elemental traces will lead you down the stairs but will not go further down when you get to the first yard shrine. The elemental traces are now headed to a floating piece of land with a lamp and a Thunder Bough on it.
Glide towards that floating land and summon an Eletrogranum to travel using Thunder Spheres and continue following the elemental traces.
After several rides on Thunder Spheres, you will find Lyudochka on top of the high mountain south of the Grand Narukami Shrine.
You need to sneak on a cliff nearby to eavesdrop on her conversation with another person, who apparently is the person who the three undercover Fatui members call “The Teacher.”
In their conversation, you will learn that Lyudochka, like most Fatui, are orphans taken in by their Lord Arlecchino. Lyudochka has Inazuman blood, making her the best person to disguise as a shrine maiden.
The Teacher will hand a note to Lyudochka, which details his next plan to cause chaos in Inazuma, specifically in Watatsumi Island.
When The Teacher leaves the area, go to Lyudochka and talk to her. But since you already know her true identity, you will need to fight her.
After defeating Lyudochka, she will reveal that The Teacher’s plan was for them to disguise themselves as people from Narukami Island, poison the lands of Watatsumi Island, and worsen the tension between the regions.
Lyudochka hesitates because this will have innocent people killed and more kids orphaned, which she herself knows how it feels like.
You will then offer to help save Lyudochka from the Fatui and ask her to hide somewhere safe. She will tell you that she will go to Yae Publishing House and stay in a small room behind their bookstore, commonly used by their proofreaders.
Go back to Momoyo’s safe house to report what you have discovered and then ask her to help you save Lyudochka.
You need to convince Momoyo to help you create a false cadaver and fake Lyudochka’s death. Momoyo will not directly agree with you, but she will tell you that you need eight chunks of raw meat, four white iron chunks, three sea Ganoderma, and one set of clearly identifiable clothes.
Collect all materials, and find Lyudochka behind Yae Publishing House’s bookstore in Inazuma City to ask her for her Fatui uniform.
Bring all the required items to Momoyo, and leave the safe house as she prepares the fake cadaver.
Wait until the following afternoon, around 14:00 to 18:00 in-game time, to end this part of the quest series and enable the next one.