If you're playing on the default story settings,
. If you have changed the start then to get this you must let a chosen inflict dazed on one of your soldiers and then kidnap them. Later you will be given a mission similar to the "rescue captured soldier" mission from the base game, but with a much higher emphasis placed on stealth (bring a reaper if you have one). Rescue the captured soldier from the holding cell and extract, and it should unlock.