When you return to Paititi after
you will receive a green suit that allows you to speak to cultists. After progressing a bit in the story you will find
and he will tell you to help
this is a secondary mission that can be easily missed if you decide to progress a little bit further. Distract the guards with music, and tell the villagers to the side to help you open the door. Once you open the door and escape with Hakan the achievement will pop and you will be rewarded with the reinforced knife.