If you’re fighting a creature you can’t see—when a creature is invisible, you’re blinded, or you’re fighting in darkness you can’t see through—you have to target a square rather than the creature. You also have to figure out which square to attack. Here’s how it works.
Invisible Creatures And Stealth[]
If an invisible creature is hidden from you, you can neither hear nor see it, and you have to guess what space it occupies. If an invisible creature is not hidden from you, you can hear it or sense some other sign of its presence and therefore know what space it occupies, although you still can’t see it
Make a Perception Check[]
On your turn, you can make a Perception check as a minor action to try to determine the location of an invisible creature that is hidden from you.
Pick a Square and Attack[]
Choose a square to attack, using whatever information you’ve gleaned so far about the target’s location. Roll the attack normally (taking the −5 penalty for attacking a creature that has total concealment). If you pick the wrong square, your attack automatically misses, but only the DM knows whether you guessed the wrong square or your attack just missed.
Close or Area Attacks[]
You can make a close attack or an area attack that includes the square you think (or know) the concealed creature is in. Your attack roll doesn’t take a penalty from the target’s concealment.
Examples[]
Aiden, a level 10 Fighter, is fighting with Lucrezia, a drow sorceress who becomes invisible. The sorceress attempts to move away from Aiden, and makes a Stealth check of 19, moving 2 squares to his left.
Aiden, on his turn, makes an active Perception check as his minor action, and rolls a 29. He knows that she has moved 2 squares to his left. He uses his move action to move adjacent to her. He then attempts to bring his broadsword to bear against her AC 23. He rolls a 12, adding his attack bonus of +14, but her concealment bonus brings his attack bonus down to a +9, making his attack total 21, instead of 26. Aiden misses the sorceress.
Aramis, a level 10 Wizard, has been blinded by a Beholder. The beholder, with his low speed, has only moved 1 square forward. He rolls a Stealth check of 22.
Aramis, on his turn, makes an active Perception check as his minor action, and rolls a 21. He knows the Beholder is still in front of him, but not precisely where. He casts his Burning Hands spell, a blast 5 attack, on the ground in front of him, against the Beholder's Reflex defense. He rolls a 15, and his bonus to the attack, without concealment, is +14, making his attack a 29 vs. the Beholder's 24 Reflex defense. The attack hits, scorching the Beholder.