This is a Random Event. Occurs in:
Navigating the fog blind, you practically bump hulls with a Mantis ship. They hail you: "Pah! This transgression will be overlooked. Nebula, very dangerous. Next time, humans all die."
- There won't be a next time. Open fire!
- Fight a Mantis ship (default rewards).
- This place is dangerous enough. Move on.
- Nothing happens.
This event is called "NEBULA_MANTIS_CHOICE" in the datafiles.