Outriders is still a somewhat under-the-radar game, but it has a loyal fanbase. Developer Square Enix can be thankful for this game as it is their most popular and most viewed steam game.
The secret quest takes a bit of time to complete as you will have to find three different monoliths that are located in various parts of the map. The locations for the three are pictured below. Once you find them, they will be glowing blue to help you spot them easier.
The first one all you have to do is travel out of your camp and go to the left. If you follow the path to the left you will find it.
The second one isn’t as easy to find. You will need to travel back to the main path and go to the main open room that you passed to find the first one. When you’re back here, you will follow a path to the right that will lead you up a hill and where the second monolith.
The last one you will need to travel to Wrecked APC and cross the Monolith Bridge, obviously. Here you will have to fight any emotes that you come across, and once you finish that you will be granted access to the final monolith.
After retrieving all three monoliths, you will have unlocked the secret challenge. The challenge is to go to the Cliffside Path and go through a door that contains a chest of loot. Once you open it you will receive your legendary item and will have completed the quest.
Following these steps will grant you the access to the Forgotten Quest. Good luck!