The are 4 codes for the artifact ending. Each one leads you to a place to find a Memory piece. I've been able to find 3/4 and have no idea how to commence... There is something about Marie, but I have no idea where to find this...
Its simple really. After getting 3 memory pieces, play through the game as normal. Then when you get the key to the basement, unlock the door. Then a prompt will appear asking if you want to enter or not. Select no, then you'll head outside and walk straight down. Keep going until a cutscene appears. After it ends, you finally get the Artifact ending
(Note that you need to actually make the call in order to have it activate first. Otherwise the prompt will not appear. Same goes for the other Memory pieces for the Artifact Ending)