Enemies that you won't be able to meet on the battlefield in a normal manner - they won't be patrolling the streets, they aren't dropped on it as part of reinforcements, they won't run out of an alien facility that you are currently attacking.
Those enemies have a nasty camouflage ability, allowing them to take the form of a normal civilian (which is why they are mostly stumbled upon during the Retaliation-type missions, in which you have to rescue civilians from an alien onslaught). You won't be able to recognize them until one of your soldiers gets close enough to a disguised enemy (or you won't use scanning abilities - which will be discussed further in the chapter). When that happens, enemies will revert to their true form and will attack their nearest target.
Their biggest threat comes from the fact that they reveal their true form only after the round of the player has ended, allowing them to attack immediately afterwards - which makes them extremely dangerous, especially during the first weeks of the game, where they can one-shot most of your soldiers. Fortunately, the player is not defenseless against them - by following a number of rules, you will be able to take them out effortlessly.
If you are expecting those enemies to appear on the battlefield (or if your soldiers are on a Retaliation-type mission, in which you're saving civilians - there's a 100% chance of meeting them here), you should remember about:
- You can remove their disguise by using scanning abilities (of the Specialist class) and items - the same that you use for reconnaissance. If the enemy is in the range of the scan, it will force him to revert to his true form - but the enemy will be able to attack only during the alien phase.
- You should always leave one or two soldiers in the Overwatch - the enemy has 0 defense (regardless of the difficulty level) and is unable to hide behind covers, making almost all shots against him a certain hit. Faceless' health bar is relatively large, but they lack any armor - the mentioned soldiers left in the Overwatch should be able to get rid of the enemy.
- Never position your soldiers on the roofs of buildings, or near cars. In the former case the enemy will be able to destroy the floor, dropping your soldier to the ground and dealing fall damage, whereas in the latter one will simply cause an explosion of the vehicle. Both of those scenarios can end in your soldier(s) dying.
- By keeping your soldiers in a distance equaling their sprint, they should be completely safe from the enemy attacks, as the Faceless won't be able to reach them and attack during the same round.