This section of Call of Cthulhu guide describes the last, fourteenth chapter titled "Coastal Cave" and then "Alabaster Point". The main protagonist catches up with Sarah Hawking and has to make a decision that will affect the fate of the world.
After regaining control over your character, march along the only available path. Enter the cave and go deeper within, until another cutscene activates taking you to a new location.
Carry on and take a look at the fresco painted on the wall to the left. Approach and examine it to unlock the Shadow out of Time achievement - provided you examined previous frescos. Continue walking.
Upon reaching the circle of candles, a cutscene featuring Marie Colden with initiate. When talking to her, you will get the chance to select the R'lyehian dialogue option three times. After a short while, the conversation will end and you will be able to continue.
Continue through the caves and climb down the ladder. Ignore the boat - instead, turn left and continue along the corridor. A loading screen will be initiated, and the chapter name will change. After a while, you will be able to control the main protagonist again.
Go a bit forward to activate another cutscene. Two versions of the main protagonist will, in turn, talk to Charles Hawkins. When the conversation ends, continue along the corridor, then go upstairs to the final location in the game.
After going a bit further, the last animation in the game will start. There are several dialogue options to select - they affect the ending of the game. Which options are available depends on the choices made by the main protagonist - this means you cannot unlock all of them with one playthrough.
Note - during the final sequence, the last R'lyehian dialogue option will be available to choose. Doing so will unlock the Fluent in R'lyehian achievement.
Selecting the ending activates the last cutscene and ends the game. Congratulations, you have completed Call of Cthulhu. For more information on the available endings in the game, see the "Endings in Call of Cthulhu" chapter.