Can you be flanked by invisible opponents?

[aboyd]

As a DM I would rule that the visible attacker gains no bonus but the invisible attacker gains a a larger bonus as his target is completely unaware he is there. Once the invis attacks and becomes visible the flanking bonus becomes normal.

This.

 

[Runestar]

Flanking seems more than just standing there. You are likely assumed to be actively doing something to distract/stymie/harry the person being flanked. An invisible person can still be doing all this. Maybe a branch conveniently strewn in his path to trip him up as he moves?

This causes the enemy to sense that something is amiss, but he cannot put his finger on it. He may not be able to deduce that the source of the problem is an invisible foe, and attribute it to some other problem (perhaps simply bad luck?). He becomes flustered and feels uneasy. As a result, he cannot fully concentrate on the foe he is fighting. The drawbacks of his lapse of concentration simulate that of being flanked.

For all intents and purposes of the rules, we simply assume that what he does is enough to stay within the limits imposed by invisibility and not break them. I do feel that certain aspects of the rules were simply meant to be adhered to, and the designers did not want us to spend too much time and effort try to make sense of them. How can an invis foe threaten/flank? He just does. How exactly? I don't know, nor do I care. That he can is good enough for me.

Look at it this way. Flanking forces the target to focus on two different foes and thus make himself a little more vulnerable to each. If I do not know someone is behind me at all then I am only focusing on the person in front of me. So the person I see gains no advantage till the invisible flanker behind attacks me and draws my attention to the fact that I am flanked.

Personally, I see this as a sort of slippery slope.

If I am able to rule that I can opt not to be treated as being flanked by someone simply by treating him as if he were not there, this could be very detrimental to rogues. Imagine if an enemy were being flanked by a rogue and a fighter. The enemy could easily opt to ignore the fighter, and focus all his attentions on the rogue. The result is that the fighter gets a larger combat bonus (which is not so relevant) while the rogue is unable to deal sneak attack damage (because he is treated as not flanking).