Put the Beast Down

 is a quest in the quest in the add-on Dead Money.

Quick walkthrough

 * 1) Gain access to the Casino electrical closet and restore power to the rest of the Sierra Madre.
 * 2) Retrieve the Emergency Door key to the Cantina Madrid kitchen.
 * 3) Access the Cantina Madrid kitchen via the Emergency Door in the Casino.
 * 4) Confront Dog/God and convince him to back down- one way or another.

Detailed walkthrough
The key for the electric switch is on the semi circular bar up the stairs to the left as you go through the casino door. It is not a key key, it looks like a piece of paper or a clipboard, it is where the marker points but it's hard to see, (lot's of people have been saying it's a random marker and they can't see anything, but it's there.)

Shutting off all three gas valves isn't incredibly difficult: turn off the one near you as you enter the area, then get the one behind Dog/God's back in the room on your left, then, as he is walking toward the far end of the kitchen, get to the last switch.

Bugs
If you turn off all of the valves before dog kills himself, Dean and Christine's collars do not explode.

The key is a damn invisible peace of paper, that takes a fucking hour to find