Serious Sam 4

Mission: When in Rome
First side mission in the level.

The blue mission marker is pretty obvious and its reading "Gabriellas Knight" and the reward is the Rage serum.
Make sure to follow the blue markers and fight your way to the warehouses.
Make your way inside and you will find a locked door that will unlock once you cleared the scorpions inside the room. The door only unlocks when you go up to it after the room is cleared.
After going through a cutscene will start and the cheevo should pop.