Benefit: In melee, every time you miss because of concealment, you can reroll your miss chance percentile roll one time to see if you actually hit.
An invisible attacker gets no advantages related to hitting you in melee. That is, you don’t lose your Dexterity bonus to Defence and the attacker doesn’t get the usual +2 bonus for being invisible. The invisible attacker’s bonuses do still apply for ranged attacks, however.
You take only half the usual penalty to speed for being unable to see. Darkness and poor visibility in general reduces your speed to three-quarters normal, instead of one-half.
Normal: Regular attack roll modifiers for invisible attackers trying to hit you apply, and you lose your Dexterity bonus to Defence. The speed reduction for darkness and poor visibility also applies.