Back 4 Blood has quite a few chapters throughout its four-act story and in each one is one secret denoted as a golden skull trophy. Interacting with them will earn you an achievement or trophy to add to your profile as well as an in-game Accomplishment. Here is where to find the secret in Remnants and earn the Extra Credit achievement or trophy.
To find the Remnants secret, you will need to be playing on Making the Grade. In this mission, you will be exploring a high school and fighting a lot of Ridden. Eventually, you will come to a gymnasium and need to power up a generator to pull out some bleachers and escape through a window. The secret is soon after this section.
After the Ogre falls through the ceiling, go up the bleachers, and you will be on the school rooftop. Continue along the path until you reach a set of stairs. Do not continue along the route, or you will drop down into an area where you can’t get back up and have to start the mission over for another chance at the secret.
Before you continue, turn to the right, and you will find a tent in the corner filled with Ridden tentacles and our golden skull trophy. Walk up to it and interact with it to unlock the achievement or trophy.