For each of the 3 alignments, there are 4 doors. In order to unlock this achievement, you must try to use a door that does not belong to your alignment. (e.g. Dark path person trying to enter a Light Path door) When you attempt to enter, it will ask you to buy one of the masks for 8,500 souls. Do this and the achievement will unlock. Here are the locations of the doors:
Light Path:1.) Ruined Cliffs > The Battlements > White Rock Chamber
2.) Ghostfang Wildlands > Wisp Grove > Whitestone Gardens
3.) Arkans Moors > Lost March > Illuminated Path
4.) Valley of the Dead > Skull Pit > Crucible of Law
Void Path:1.) Ruined Cliffs > The Battlements > Puzzling Path
2.) Ghostfang Wildlands > Shadow Glenn > Times Doom
3.) Arkans Moors > Well of Gloom > Cave of Betrayal
4.) Valley of the Dead > Temple Gates > Eternity Vault
Dark Path:1.) Ruined Cliffs > The Battlements > Dark Mausoleum
2.) Ghostfang Wildlands > Heart of Kul > Well of Cinders
3.) Arkans Moors > Gor Loch > Shadow Rock Tomb
4.) Valley of the Dead > Skull Pit > Pains Embrace
Here is a video if you prefer that over the text: (credit to user SilentRich Achievement Guides for video)
Good luck and let me know if there is anything I could add to improve the guide!