This is a story progressing achievement, as opposed to what it may sound like (but I guess the TA ratio kinda gives it away). You don't need to do anything apart from playing the main story in the DLC.
It pops right after you exit the big room with the puzzle that involves moving towers and you wallrunning between them. You need to open a big door by means of handhold-switches, if that rings any bell.
Below is a video, to clarify. Credits go to mattb101586.