Sunless Sea: Zubmariner Edition

You can get this in Fallen London by doing the "Carouse in Wolfstack Docks" event when you have a free day each visit. It may take a few tries at this event before you get the right outcome. Choose one of the named options (not the I can't remember their name option).

Later on as you travel you will get an event asking whether you want to keep the locket or throw it away. Keep the locket and the achievement will pop.