Does Whirlwind Attack allow you to attack an invisible creature you didn't know was there?
Take a medium fighter (F) under the effects of enlarge person, so they are now large and have a natural reach of 10. He is fighting goblins (G) and somehow two invisible ones (I) have joined the fight behind him without him knowing about them.
G · G · G · · · · · · · · · F F · · · I F F · · · · · I · · · · · · · ·
So when the fighter goes to Whirlwind Attack, he first goes to attack the 3 goblins he knows about, but would he be allowed to also attack the two he didn't know about? Whirlwind Attack only seems to care about being within reach. So are the attacks granted and done with concealment or are they denied because you didn't know to attack those squares?
Benefit: When you use the full-attack action, you can give up your regular attacks and instead make one melee attack at your highest base attack bonus against each opponent within reach. You must make a separate attack roll against each opponent.
Also, would you now know about the invisible creatures?