The Replacement's Secret is the second part of The Very Special Fortune Slip World Questline. It starts as soon as you complete the Shummatsuban Operations World Quest.
Momoyo will give you a Bamboo Slip, which you need to hand over to Gendou Ringo at the Grand Narukami Shrine. She will, in turn, give you a Special Fortune Slip. Once that's done, follow the quest marker to the waiting spot by the cliff and change the in-game time to anytime between 21:00-23:00.
A cutscene of Gendou Ringo leaving the Shrine will prompt. You need to use Elemental Sight to follow her traces.
The traces will lead you to a small platform with a Thunder Sakura Bough, which you need to interact with to pick up an Electrograna to traverse the Thunder Spheres to lift you upward. Doing so will bring you to the Fatui's secret meeting spot.
A cutscene of Lyudochka and the Teacher will play out. After, speak to Lyudochka and engage in a battle with her. She'll turn into a Fatui Electro Cicin Mage. Defeat her and prompt a dialogue to obtain the quest item called Final Intructions.
Head back to the Shuumatsuban's secret hideout and relay the information to Momoyo. She'll agree to create a fake cadaver to help Lyudochka escape. You need to gather the following materials:
- x8 Raw Meat
- x3 Sea Ganoderma
- x4 White Iron Chunk
Follow the quest marker to the back of the Yae's Publishing House in Hanamizaka, Inazuma City to obtain Lyudochka's clothing.
Once that's done, report back to Momoyo and hand her all the materials. Leave the secret hideout until you reach the glowing spot outside, then change the in-game time to 14:00-18:00 the next day to complete this World Quest.
To unlock this questline, check out our guide on How to unlock The Very Special Fortune Slip World Questline (SHUUMATSU GAIDEN Achievement) in Genshin Impact.