Unseen
From Titan's Teeth
An unseen creature gains a +2 bonus on attack rolls against sighted opponents, and ignores its opponents’ Dexterity bonuses to AC (if any).
Attacking Unseen Creatures
Unseen characters benefit from total concealment, meaning opponents must guess the square that the character is in, and even on a successful attack suffer a 50% miss chance.
A creature with the Blind-Fight feat has a better chance to hit an unseen creature, reducing the miss chance to 25%. A creature with blindsight can attack (and otherwise interact with) creatures regardless of unseen state.
Detecting Unseen Creatures
If an unseen creature strikes a character with a melee attack, the character struck knows the location of the creature that struck him (until, of course, the unseen creature moves).
If the unseen creature has a reach greater than 5 feet, or is using a ranged weapon, the struck character knows the general location of the creature but has not pinpointed the exact location.