One of the first things that you will notice as you wander around Croft Manor is that many of the doors are locked, barring your access to much of the Manor’s secrets. The item that you need to unlock all of these doors is the Master Key, and if you follow this guide it will lead you right to it so you can explore the entirety of the Manor.
First of all you’re going to need the flashlight, which you can find on a cabinet near the entrance to Croft Manor. Once you have that you’ll be able to navigate the dark passageways leading to the basement, which are accessible through a door in the hall leading to Croft’s office. If you’re having trouble finding it, use Lara’s Survival Instinct to spot the wall with several hand prints on it.
Navigating down and through the basement and moving around some shelves will reveal a flooded and rundown area that Lara has to crawl through, though it is pretty straightforward. It will eventually lead you to the library via a ladder and then a moving bookshelf, and on one of the desks in the library you will spot an item. This will be a journal entry, but it will also come with a lighter. Head into the back of the library and move the ladder so that you can climb up to the top floor (moving around some obstacles along the way), picking up the map at the end of the walkway. While this map initially appears to only show a key, holding it to the fireplace in the library (after lighting it with the lighter) will reveal the full picture.
This will reveal that the key is hidden beneath the trunk that is just behind the fireplace in the library, and all you have to do is move it to reveal a case in the ground. Open it up and you now have the Master Key. This item is vital to completing this DLC, so you should be well on your way now of getting through it.