This is story related when you get to the end of Chapter 7: The Keeper.
*** Spoiler - click to reveal *** You get this at the end of Chapter 7 when you need to turn off the 3 valves and kill the keeper with a trap. The switch will be right next to you as you run out the door after repairing and turning off the final valve.
There are also great guides to get
Along with
What's in the box?!