A unit that has Stealth can’t be targeted by Ranged or Psychic attacks as long as they have Cover. In Relic Knights, Cover is not an inherent feature of a terrain piece that a unit can claim by standing in or close to it. Instead, Cover is a function of drawing line of sight, and it only exists when line of sight is drawn from an attacker to a target, and the attacker need only draw one unbroken line from anywhere on its base to anywhere on the target’s base. It means units with Stealth often have a difficult time claiming its benefit as they can’t just place themselves by a terrain piece which always gives them Cover. Enemy units can usually position themselves in such a way that they can get a clear line of sight.
The Darkfield is a 4pt boost that grants Cover and Stealth to friendly units within 3″. It is the only instance in the game where Cover is an inherent feature rather than a function of line of sight, so it means units stood close to a friendly Darkfield always have Stealth, even against attacks that do not require line of sight and thus would normally bypass Stealth. It makes the Darkfield an invaluable purchase for important units that lack the durability to survive long on their own.
There is a downside to hiding a unit in a Darkfield, in that its range of movement becomes very limited if it wishes to continually benefit from it. Consequently it is best used by static units and by squads. In the former case, the likes of Togan & Cecilia and CSM Alex-117 can be set up in a Darkfield along with supporting units that trigger their Officer ability, and be used as a static gunline whilst all the units benefit from the Darkfield. A Serpent Priestess can often afford to remain static and be used for her Compel and esper generation without needing to have her run objectives. A Darkfield can also be used to protect one of your objectives, which is especially useful if your opponent has flipped Tear It Down, or if you need to interact with them for your own victory conditions.
Squads are the other big beneficiary of Darkfields. As only one member of the squad need be in the area of the Darkfield for all to claim the benefit, then they can be strung out in a line to collect and/or deposit tokens. They can leave the Darkfield in their initial move if needs be and return to it with their follow-up move. As squads can be very vulnerable to damage from collisions and falling, a Darkfield can make them a viable choice where they otherwise would not be. Act As One squads derive even more benefit as they can cover all of the board whilst still benefiting. It means Suspect 7 can freely roam the board with no fear of Ranged or Psychic attacks.
Dealing with a Darkfield can be very difficult, but it can be done. As it benefits from its own buff, then the Darkfield itself can never be targeted by Ranged or Psychic attacks. If you want to destroy it, you’re going to have to get into melee with it. It has 8 Health and 2 Armor, and as you won’t want to be wasting too many activations on it then ideally you’ll want to be able to take it out in one go, so having a mobile unit with a high damage Melee attack is a very useful anti-Darkfield choice. It can also be directed at the units hidden the field. If you can do it, killing CSM Alex-117 is better than destroying the Darkfield which is hiding him.
It is possible to switch a Darkfield off by switching off buffs. The Chaos action, Miscommunication, can be placed directly under a Darkfield preventing any enemy units gaining Stealth from it until they can remove it. Moving it is another option as this can reveal the units hidden within it, at least temporarily. It can’t be moved by Ranged or Psychic attacks for obvious reasons, but Melee attacks can, as can the Essence action, Stand Back, as it’s a Support action, and Viper’s Saboteur action too.
Even if you can do any of these things, always consider the cardinal rule of Relic Knights first and ask yourself whether you actually need to do it. If you can complete your victory conditions sufficiently quickly even if your opponent is using a Darkfield, then trying to deal with it will only slow you down needlessly.