One of the biggest novelties in the XCOM 2 is the Concealment mechanic. While it is active, your soldiers will have a small icon representing a hooded person next to their names. It's not the icon that matters though, but the thing it represents - as long as it's active, your units will be practically invisible to the enemies. Aliens simply won't have any idea about the existence of your units and they will simply mind their business.
Concealment is given to units at the beginning of most missions (aside from the Retaliation-type ones). While on the battlefield, your units will be able to approach the enemies from a really close distance before they are noticed. It's represented by special, red icons on the ground, which show you the "sight range", spreading a few squares in each direction from each enemy unit. If one of your units steps on that field, or fires his / her gun, the concealment will be broken and the enemy will be able to see all of your units.
A thing worth mentioning is the fact that the Ranger class has access to a number of abilities which modify (for that single unit) the concealment mechanic. The Phantom ability allows the unit to remain concealed (provided that it wasn't that unit that was seen by the enemy) even when the rest of the team was seen. Additionally, the unit will be able to enter concealment even during those missions, where it's normally unavailable. Conceal ability allows the Ranger to enter concealment once more. Both of those abilities make the ranger a powerful reconnaissance unit, allowing the soldier to sneak up on an unsuspecting enemies.
You should remember about a number of things:
- Concealment is broken when one of your soldiers performs almost any action (like throwing a grenade, attacking an enemy and even hacking), or if they simply make a lot of noise - by bursting through the door, or by smashing a window.
- You can approach the enemy from an extremely short distance, such as 1-2 squares, provided that there's a cover, or an obstacle between both units. The enemy won't be able to notice your units if there's no direct line of sight between them, but if they are approached from the side, of from behind, it won't help your units and they will be noticed.
- Concealment phase allows your units (in moderation) to move around the map by sprinting - the chances of stumbling upon enemy units are rather small.
- Some abilities won't break concealment - you can use most non-offensive abilities, as well as scan the area, or even heal your units. Hacking in general is quite dangerous, as most of the actions you can perform while doing it (like taking control over a robotic enemy) will break your stealth.
- Never, ever break concealment to attack an enemy from a very long distance - if you are able to sneak up on him, you should always make use of it!
How to use it
Concealment phase gives you a tremendous tactical advantage over your enemies - it allows you to look around the area and locate enemy patrols, or mission objectives, but above all that it will give you an opportunity to prepare a deadly ambush. As enemy units won't be able to notice your units before they are in extremely close range, you can approach them and do a surprise attack.
To do so, scatter your soldiers around the area, focusing on their preferable positions (such as a high elevated spot, with full cover - a dream come true for every sharpshooter), place most of them in the overwatch and use a single soldier to attack the enemy. A good thing to initiate the ambush with is a grenade - it will hurt a number of enemies, and at the same time will demolish any nearby cover, allowing your units to have an increased chance to hit against them. You can also leave more than one soldier out of the overwatch - it will allow you to finish off any survivors.
A couple of things that you should have in mind before preparing an ambush:
- Remember about merits and flaws of each class - a sharpshooter will be the most effective while fighting from a long range, whereas a ranger will benefit from attacking enemies from a few tiles at most.
- Even if you break concealment, it does not mean that the enemy will flood your position with soldiers - there is a good chance that they will simply continue their own actions like nothing happened, but you will have a harder time to sneak up on them.
- If it's possible, you should place your soldiers around the group of enemies - the more sides you attack from, the bigger the effectiveness of the ambush, as enemies won't be able to find any cover.
- If you aren't entirely certain that there aren't any more enemy groups in the area (or if a group is not really close to your squad), never do a surprise attack - it might end up badly when suddenly two, or even three groups of enemies come out of nowhere.