I am having trouble finding the key to the house described in Sealed Missive quest. It just tells me that I need the proper key to unlock the door, but I have no idea where the key is. I went back to the place where the dying monk gave the quest but his corpse is no longer there, so I no longer have any idea how to proceed. Help is greatly appreciated

would this help: https://pillarsofeternity.gamepedia.com/The_Sealed_Missive