Enter the Freethinker Hideout (raid zone) and proceed to the breakable wall four rooms to the left as you enter the zone (in the south wing ( 65, 4, 49 ) Copy/waypoint 65, 4, 49).
Use the barrel at ( 59, 0, 12 ) Copy/waypoint 59, 0, 12 to destroy the breakable wall and open the hidden room but don't move a barrel without knowing what you're doing or you can screw up the zone.
Beware of the werewolves.
When the wall is blasted, one werewolf will pop at the door opening to the hallway with the breakable wall and three more appear further down the other corridors.
They start pathing within a couple minutes and traverse the entire corridor system including the room hidden by the broken wall.
Kill the roaming werewolves, then enter the room and click on the middle chest in the hidden room to retrieve the Blackwater Mask.
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