Story related, which should pop at the very first check point of chapter 6, when you reach the ritual chamber, which has a fire/lava fountain in it. A short video sequence will start and the achievement should pop after that (right after Mac says "I'll be damned")
I wrote 'should' as it's currently buggy and does not unlock for everyone.
To re-trigger the achievement restart your game, select chapter 6, choose Keegan as your character and the same difficulty as your initial playthrough (or experienced as this worked for me, but was also the difficulty of my initial playthough).
It may take some tries, but will unlock eventually.