This is considered the most complicated achievement to acquire, mainly because the tasks require you to mess with the game's files. Open your Steam library and right-click The Stanley Parable in your games list, then select the option that says Properties. Select the menu at the top that says Local Files, then select Browse Local Files. This should take you to the game's files. Click on the folder named thestanleyparable, then another folder titled cfg.
Cthulhu Ending #13
Press the red button twice to reveal a 3 button panel.
Enter the code 312213 in the new panel to reveal a lever.
Pull the lever to the left to reveal a ten-digit panel.
Press the red button six more times to reveal your screwdriver.
Drag the screwdriver to the small panel on the bottom right side of the monitor and press the tiny red button that is now visible in order to reveal a nine-digit display.
Enter the code 011235813 in the top right 10-digit keypad.
A pillar and four spires shoot up out of the ground, and Cthulhu is on the pillar.
Stanley Parable Keypad Code