Being Hidden confers various benefits; most significantly Hidden fighters cannot be targeted by attacks while Hidden. Fighters may begin a battle Hidden and / or become Hidden during a battle where and when indicated by scenario write-ups, or via a skill or similar special ability (such as Shadow), or via a Tactics Card where relevant.
Hidden status is indicated by either placing a Hidden token on a Hidden fighter's fighter card, or next to the fighter on the battlefield. Some scenarios allow a Hidden token to be placed on the battlefield instead of the fighter's model and moved around as if it were the fighter to represent their position on the battlefield.
Once a Hidden fighter has been detected, they lose their Hidden token. If a Hidden token was being used to represent where the Hidden fighter was, the fighter is set up onto the battlefield where the Hidden token is when the Hidden token is removed.
Default Hidden Guidelines
Some scenarios provide variant guidelines for how Hidden status and Detect Hidden Fighters works during battles fought using those scenarios, and similarly an Arbitrator making a custom scenario of their own may decide to provide alternate guidelines for how Hidden works (or doesn't work) within their scenario. However, if no variant guidelines are provided Hidden status and Detect Hidden Fighters work as described below.
Hidden Status
Hidden: While Hidden, this fighter cannot be the target of enemy fighters (however they might still be hit by weapons with the Blast or Template traits). Additionally, a Hidden fighter's close combat weapons gain the Backstab Trait; weapons that already have the Backstab Trait gain +2 to hit when the Backstab trait applies.
Detecting Hidden Fighters
Active and Pinned fighters can make the action Detect (Basic) in an attempt to sense Hidden enemy fighters.
Detect (Basic): The fighter makes an Intelligence check and if successful detects something. What is detected depends upon the scenario or situation. Bonuses or penalties may apply, as indicated by scenario special rules or other game effects.
Hidden Fighter Detection Modifiers
Hidden fighter...
Modifier
...has a Blaze marker on them?
+2
...is carrying an Unwieldy weapon?
+1