While shooting down deranged humans and vicious vampires is what you will be doing most of the time in Redfall, there will be times when the game will ask you to put aside your handguns, shotguns, and stake launchers temporarily to explore locations in search of specific items that are required to complete certain quests.
One of the very first story quests that require you to find some items to move the story forward is the House of Echoes quest. Finding the dolls to restore the dollhouse, however, is just the start of the item-hunting you will have to do in the game, as you will need to find one more elusive item to finally complete the quest: Addison's Lab Key.
Where to Find Dr. Addison's Lab Key
Once you restore the dollhouse, you will be transported to the Psychic Mansion, which isn't any less dangerous than the mansion in the real world, as there are some enemies roaming about here as well. To leave this haunting version of the doctor's mansion, you will have to access his lab, and to do so, you will need Addison's Lab Key.
Unlike the dolls, which could be found in specific locations around the mansion, the Lab Key isn't found in any precise location, as to get it, you will have to defeat the three Sin Eaters roaming the mansion. Interestingly enough, they can be found roughly in the same places the dolls were located. The first Sin Eater can be found on the mansion's ground floor, near the kitchen; the second is in the attic; the third and final one nearby the greenhouse outside the manor. After defeating each one of them and experiencing the memories they leave behind, the Lab Key will appear, allowing you to head into the lab, learn more about what led to the incident that forever changed Redfall, and complete the House of Echoes quest.