Call of Cthulhu

Is obtained in Chapter 3 when talking to Silas the Caretaker (the conversation after he threatens you with the axe).

Technically missable, as some failed skill checks or bad conversation may cause you to have to fetch the key from the cabin. Fortunately, there is a conversation path where you don't need any skill checks.

First, on your way to the manner before meeting Silas, make sure you take the left path to the graveyard and inspect Sarah Hawkins grave -- this will unlock a conversation item that will get Silas to cooperate. From the grave, continue on the level until you encounter Silas.

During the conversation, answer as the following:

-- I work for Stephen Webster (no requirements needed for this choice)
-- Sarah Hawkins was important to you (must have inspected Sarah Hawkins grave)

He'll give you the key to the mansion after this, and achievement unlocked!

At the beginning of Chapter 3, when you start, take the first left into the graveyard and inspect all 3 graves. As you then try to go into the mansion, you will get stopped by Silas the caretaker.
This CAN be missable, but with two dialogue options, this is as easy as pie!

"I work for Stephen Webster" (No requirements needed)
Then, (Assuming you inspected Sarah Hawkins Grave)
"Sarah Hawkins was important to you"

Achievement should then unlock.
Heres my 100% Achievement guide if you want to see how, where and when this unlocks! Go to 24:20 in my playthrough 1 video.

Hope it helps!

Playthrough 1

Playthrough 2