The "Devil in Velvet" mission in "Collapsed Skyrail" is a pioneer mission in version 2.1 of the game. Although detailed steps are not provided directly, combined with existing information fragments, a rough guide for completing this task can be compiled for you:
Answer: In the first stage of the tracking mission, just have a conversation and go through the plot.
1. Witness the death of the robin and talk to Sand Gold. Black Swan joins me in the middle. Sand Gold gave me 50,000. After leaving, talk to Black Swan.
2. On the other side, Huang Quan and Sam fought against each other. At the same time, it was confirmed that the Grand Duke was indeed killed by Huang Quan, but his words seemed like his children were not dead yet.
3. The train crew agreed to cooperate with Sand Gold, and Sand Gold gave the player another 200,000, which must be said to be very wealthy.
4. Walter set off the train and talked with Huang Quan. At the same time, the real black swan arrived in Huang Quan’s room.
5. Finally, I used the music box to search for memories but heard Constance’s voice. After completing the plot, I entered the next stage.
The above is all about how to complete the Demon Mission in Velvet 2.1 in "Honkai Impact: Star Rail". For more game guides, please click on the source code website.