Dragon's Dogma 2

In order to start this quest I do believe you need to at least progress the main storyline to the point where you cross the border and reach the main city in the southern part of the map. (I also recommend being a high level and be prepared as the last part of the quest is very hard).

I started this quest by overhearing a rumor while on an Oxcart about a Phantom Oxcart. Go to the marked location to begin the quest.

You will eventually reach a point where you have to options do complete the quest. Either pretend you are a pawn and board the oxcart or follow it. To get the achievement do the pretend to be a pawn by unequipping everything.

Video guide attached if you need more information.

This achievement is super easy to get. If you already have the quest than just unequip all gear after giving captain the document. If you don't have quest exact unlock is provided in video with all other need to know information.