Those who have already played Rise of the Tomb Raider will notice that the Croft Manor can be quite a maze with its many doors. Most of them are locked containing secrets and items, and the only thing that can unlock them is through the Master Key. Here's a step-by-step guide for you to find the key and access the Manor's secrets.
Since the Croft Manor holds a lot of secrets, a lot of its areas are shrouded with shadow, literally and figuratively speaking. Therefore, you need a flashlight if you want to go around all over the vast abode. Get the flashlight from the cabinet located near the entrance of the Croft Manor.
After that, you can navigate the dark halls of the basement using your flashlight. The entrance is located in a hall which leads to Croft's office. However, it can be quite difficult to find the door of the basement itself. Use Lara's Survival Instinct to locate the wall which has several handprints on it.
Down the basement, you will find the vast Croft library but before getting there, Lara has to crawl through some flooded areas. After that, she will find a ladder leading to the library. There she will have to move a bookshelf and one of the desk to get the journal entry which also comes with a lighter.
After acquiring the items, go to the back of the library and use the ladder to climb the top floor. Be careful because there are some obstacles along the way before reaching the top. There, you'll be able to get the map at the end of the walkway. Using your lighter, light the fireplace and hold the map against the light to reveal the hidden message.
The message reveals where the Master Key is.It is inside a trunk which is hidden behind the fireplace. Move the trunk to find a hidden case in the ground and when you open it, you can find the Master Key. Get the key and complete your Rise of the Tomb Raider mission.