This is the 9th story progression achievement.
*** Spoiler - click to reveal ***After the cutscene at the end of ‘Sanctuary’ you’ll be told to speak to Rebecca to receive the ‘Encroachment’ mission, which will task you with destroying some new toys that K-Sec have been planting in the tunnels. The mission is incredibly straight forward – simply follow the marker to the 3 objectives and push each of them into the abyss, taking out any enemies along the way. The only tricky part of the mission might be when you get to the third objective due to there being more guards in the area, but if you keep moving quickly you should be able to take them down with ease. After taking care of the third objective you’ll be told to hit a switch to open a tunnel door – doing so will complete the mission and unlock the achievement.