It Came From Within Achievement/Trophy Guide
Step 1: During Act 2 you will meet Bruz who will teach you how the domination ability works and how you can take over other captains minds to use them in your army. Once this has been taught, go out and kill OR recruit the 2 warchiefs of this area. I recommend just killing them.
Step 2: Once they are dead you need to recruit some captains and then in the nemesis menu, command them by selecting
and hover over the dead warchief and select Infiltrate
. This will now start up a new side mission called Infiltration
Step 3: Head to the event and watch your captain fight another for the spot of spy.
Step 4: Now the warchiefs spots have been filled with your captains, start the assault on the fortress and the achievement/trophy will unlock instantly.
Here is a video of me doing it.