After you've helped Pearl and Duane at the Chapel during Part 2 of the game you will enter through the Altar, when you enter the room is in darkness and you're supposed to interact with the Stone Object
DO NOT DO THIS.
Firstly move around and interact with
on the 'Bottomless Pit'
Bottomless Pit
Then select the
Mason's Hammer from your inventory with
and use it on the 'Figment of the Imagination' using
Figment of the Imagination
Finally interact with the 'Imaginary Malicious Denizen' using
still with the Mason's Hammer equipped
Imaginary Malicious Denizen
The achievement will unlock once George says he's won.