The Ascension Project
Besides cover, another way to avoid attacks is to make it hard for opponents to know where you are. Concealment encompasses all circumstances where nothing physically blocks a blow or shot but where something interferes with an attacker’s accuracy.
Concealment gives the subject of a successful attack a chance that the attacker missed because of the concealment. Typically, concealment is provided by fog, smoke, a shadowy area, darkness, tall grass, foliage, or visual effects that make it difficult to pinpoint a target’s location. When one such conditions is present, and interferes with your Line of Effect, then the target has concealment. Usually the concealment bonus is indicated by the effect providing it. If not, it is up to the GM to determine its bonus.
Concealment Miss Chance: Concealment gives the subject of a successful attack a 20% chance that the attacker missed because of the concealment. If the attacker hits, the defender must make a miss chance percentile roll to avoid being struck. (To expedite play, make both rolls at the same time). Multiple concealment conditions do not stack.
Total Concealment: If you have line of effect to a target but not line of sight (for instance, if he is in total darkness or invisible, or if you’re blinded, or if there is a fog too thick to see through), he is considered to have total concealment from you. You can’t attack an opponent that has total concealment, though you can attack into a square that you think he occupies. A successful attack into a square occupied by an enemy with total concealment has a 50% miss chance (instead of the normal 20% miss chance for an opponent with concealment). You can’t execute an attack of opportunity against an opponent with total concealment, even if you know what square or squares the opponent occupies.
Ignoring Concealment: Concealment isn’t always effective. For instance, a shadowy area or darkness doesn’t provide any concealment against an opponent with darkvision. Although forms of invisibility, like the Tactical Cloak power, provides total concealment, sighted opponents may still make Perception checks to notice the location of an invisible character.
Varying Degrees of Concealment: As with cover, it’s usually not worth differentiating between more degrees of concealment than described above. However, the GM may rule that certain situations provide more or less than typical concealment, and modify the miss chance accordingly. For instance, a light fog might only provide a 10% miss chance, while near-total darkness could provide a 40% miss chance (and a +10 circumstance bonus on