AuraSeer said:
Yes, you're right, a straight line. The point is, you must pick the correct line. You need not pick a specific 5' cube, but you do need to specify a direction.
The ability does not say that you need to pick or know the direction. You shoot the arrow, and it goes in a straight line to the target. Or maybe it makes more sense with a slight wording change: it goes to the target in a straight line. It going to the target implies most strongly that the arrow 'knows' where the target is, assuming it is in range.
Say I'm standing in the center of a room with two doors, one each on the north and east walls. I think my long-time nemesis is standing behind the eastern door, waiting to ambush me. I fire a Phase Arrow to the east, with the instruction to strike "Grog the Evil." However, Grog is actually behind the northern door. My arrow passes through the east door, and continues on to the end of its range, but does not hit Grog because it cannot turn in midair.
To say it another way: when firing a Phase Arrow, choose a straight path for the arrow to follow, ignoring physical barriers. If at any point along its flight, the arrow enters the square of an identified target, make the attack roll.
To me, pick a direction means pick the end point of the path. A vector is defined by two points (as a line), but has a direction. That means I have to pick a 5' square (or cube in 3-D), and draw a line from the center of my square to the other. Any square along that line is 'along that direction'. If I pick the 5' square next to my opponent ('next to' from my perspective), then I would miss, per your interpretation.
I would say that if you are off by 90, 60 or maybe even 30 degrees, the phase arrow would miss. If it were 30 degrees, the target would have to lie within a cone effect which has a range equal to your bow. Sounds pretty reasonable to me. It is a spell-like ability, but I don't see it as shooting the arrow straight up and having it veer toward the target.
Third: No one ever said that Phase Arrow is weaker than Seeker Arrow. It's just a different ability. For one thing, it ignores armor bonuses to AC, making it very useful for special-effect arrows (like the Arrow Of Death ability of this same PrC).
If the seeker gets an attack roll within its restrictions (if the king hides inside a footlocker he would be immune), but very few shots with the phase arrow even get to make an attack roll, then phase arrow would clearly be weaker. If you consider the circumference of a circle of radius 177.5 feet would be over 1100 feet, and say each 5' is a direction, you have nearly 225 different directions to shoot from (in a whole circle). Same thing over a sphere gives you about 15800 directions. Only one 'direction' gives you an attack roll per your interpretation. I want an attack roll any time I have a clue where the opponent is.
By the time you get arrow of death, a DC20 fortitude save is irrelevant to almost everything you fight (other NPCs, outsiders, dragons, other beasties with huge # of hit dice).
Ah, okay, I didn't spot this bit before. Shall I just file you under "Sage-basher" right now, or did you want to insult me some more first?
No, the sage is the best. But anything that is in sage advice goes through the 3E rules committee first, then into the DnD FAQ. His 'rulings' are law as far as I'm concerned. He helped us out with several rulings as well, but they have not gone into the faq yet, either.
Burst effects and other things that happen at the same time as criticals do affect creatures that are not subject to criticals. The crit does not happen, but the other effect just uses the threat confirmation mechanic as a convenience. Also, fortified armor negates burst effects and, more importantly, vorpal, and other crit mechanic effects. Negating a critical hit is stronger than being not subject to them; 'negating' is active defense, while 'not subject' is passive. The active defense if 'better'. But as long as this does not make it into sage advice or the DnD FAQ, I would not try to pass it off as Truth.
-Fletch!
lol 
