Two party members—one visible, one invisible—flanking an enemy archer. The invisible PC hasn't attacked the archer, and the archer is unaware of the invisible PC.
The archer wants to take a 5-foot step away from the visible PC so he can make a full attack against the visible PC without provoking attacks of opportunity, but the invisible PC occupies the square the archer tries to step into.
The GM worked around this problem this time, but we'd like to know what the rules say about this situation.
What happens when a creature tries to take a 5-ft. step into a square occupied by an invisible, undetected foe?
Best Answer
As per RAW:
Source: Special Movement Rules
So I think that 5ft movement will not happen. Your DM will probably justify that the archer will feel some kind of force pushing him back, and he will then know there is something in that square (total concealment for attacks still applies). Also, you're not entitled to AoO, because a 5ft step doesn't provoke them.
Even if RAW is kinda clear in this case, your DM may rule otherwise. One possible resolution for this action would be the archer initiating an accidental Overrun combat manuever, with some kind of penality.
Source: Overrun
The archer would then roll CMB, and receive attacks of opportunity in the process. This may better reflect the situation.
EDIT: Added sources, as per comments request