Forgotten Chapel is a secret side quest in Outriders. It is considered a secret quest because the player isn't explicitly informed about it when they get close to starting it. If players want to Platinum the game or unlock all achievements, they need to finish this quest.
Go to the Canyon of the Grand Obelisk
Towards the end of Outriders, players will eventually arrive at a region named the Canyon of the Grand Obelisk. The Canyon of the Grand Obelisk is the exact location that the secret quest Forgotten Chapel is located. It's not a time-sensitive quest, so players who have beaten the game can go back to this location and complete it.
Find and activate the three pillars
The Forgotten Chapel quest requires no dialogue; instead, players must interact with three pillars located at the Canyon of the Grand Obelisk to begin the quest. None of the pillars are surrounded by unique enemies, which makes completing this quest easier.
The first one is located directly to the left of the base camp. If players go to the Canyon Entrance section of this region, walk down the path to the left rather than climbing the stairs initially, and your character will be able to interact with a pillar. Your character will then place their hand on the pillar, causing it to sink into the ground.
Players can get the second one by continuing to climb the stairs. An accessible path is found near a gate to the right. Players need to walk down this path, then activate the second pillar.
The final pillar is located to the east of The Monolith. There is a third pillar behind the Monolith's temple, just up the stairs. Players will receive the Forgotten Chapel quest after they interact with this last pillar.
Find the Forgotten Chapel and collect the Legendary weapon
Make the Forgotten Chapel the main quest and follow its objective marker. Players will come across a door with Symbols from the pillars. All players have to do is go inside.
Inside is a chest with a Legendary weapon. Players can do the Forgotten Chapel quest only once, but players can start it even after completing the main game. While The Forgotten Chapel quest is the quickest way to get a legendary weapon, it is not repeatable like all other quests in the game, so players can only get the legendary weapon once.