Concealment

Over the course of combat, characters may have concealment due to various effects against attacks. Concealment provides a character with a flat percentage chance for an attack to miss. This percentage change is determined by the type of concealment provided.
To determine whether a target has concealment from a ranged attack, choose a corner of the attacker’s square. If any line from this corner to any corner of the target’s square passes through a square or border that provides concealment, the target has concealment.
When a character makes a melee attack against an adjacent target, the target has concealment if their space is entirely within an effect that grants concealment. Characters making a melee attack against an adjacent target, that doesn't have concealment, from concealment are not affected. When making a melee attack against a target that isn’t adjacent to the character, use the rules for determining concealment from ranged attacks.
In addition, some magical effects provide concealment against all attacks, regardless of whether any intervening concealment exists.

Partial Concealment Miss Chance

Partial concealment gives the target of a successful attack a 20% chance that the attacker misses. The character makes the attack normally—if the attack hits, the target must roll a d100, and if the roll is within the percentage of miss chance they avoid being struck. Multiple concealment conditions do not stack.

Concealment and Stealth Checks

A character with concealment may make a Stealth check at any point that they benefit from concealment.

Total Concealment

If a character has line of effect to a target but not line of sight, they are considered to have total concealment from the character. The character can not target another character that has total concealment, though they can attack into a square that they think the character occupies. A successful attack into a square occupied by a target with total concealment has a 50% miss chance (instead of the normal 20% miss chance for an opponent with concealment).
Characters can not take attack of opportunities against targets with total concealment, even if they know what square or squares the target occupies.

Ignoring Concealment

Concealment isn’t always effective. An area of dim lighting or darkness doesn’t provide any concealment against a character with dark vision. Characters with low-light vision can see clearly for a greater distance than other characters with the same light source.

Varying Degrees of Concealment

Certain situations may provide more or less than typical concealment, and modify the miss chance accordingly.