Re: Visible Warrior Invisible Sword
 

I agree that somewhere between -4 (they step into measure and attack, you don't know what weapon they have or how long it is) and +0 (you are in close combat with them) seems reasonable.

In real fighting against things which move too fast to see and respond to such as spear thrusts, a common tactic is finding the sword: put your weapon between your body and their weapon, close to it or touching and with an advantage in leverage, so you can feel what they are going to do. If I ever reworked the weapons skills, I would probably have a class of skills like Rapier and Spear which assume that you lead with the weapon point-in-line. Those skills would probably have lower penalties to defend against this hypothetical invisible weapon.

Re: Visible Warrior Invisible Sword
 

Any normal person would be more likely to blow at least a round staring, and muttering "What the hell is going on there?"

Re: Visible Warrior Invisible Sword
 

Unless you are informed in advanced about the invisible person or weapon, you'll have to use the surprise mechanics.

Re: Visible Warrior Invisible Sword
 

In a campaign that allows a bit of sillyness, I would introduce the invisible swordsman by reputation. Then have the PCs meet him and only then realize he isn't the "invisible (swordsman)" but the "(invisible swords)man".