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This question came up when the spells wall of fire and blade barrier were cast and we weren't sure how to handle it. Described below are the issues we had, but first the relevant portions of the spell descriptions:

Wall of Fire has an Effect entry that says, in part, "opaque sheet of flame up to 20 ft. long/level". This sheet is hot enough on one side to cause damage, thus placement of the sheet can be very important for tactical reasons. It's pretty clear (although a derivation is beyond the scope of this question) that the "sheet of flame" must start at a given grid intersection and continue straight to another intersection.

The Blade Barrier spell also contains an Effect entry that says, in part, "[a] wall of whirling blades up to 20 ft. long/level".

These are the issues we found:

  1. If the sheet from the wall of fire spell is razor thin, as the word "sheet" implies, how far does the heat extend when the sheet partially passes through a square? For example, say 50% of the square is on the cool side of the sheet and 50% is on the hot side -- which squares cause heat damage?
  2. For blade barrier, how thick is the wall? Should it occupy an entire 5-foot square? If it did that, wouldn't it be an Area instead of an Effect, since the former is used to describe spells that occupy squares on the grid? And how would that affect the ability to create a ring out of the blade barrier? The description uses the phrase, "vertical curtain of whirling blades" so should it also be razor thin, just as the wall of fire is?
  3. Blade barrier says if the barrier is evoked so that it appears where creatures are, each creature takes damage as if passing through it. But if it's razor thin and the barrier is placed along the X or Y axis of the grid, then it doesn't actually occupy any squares, does it? Would creatures on either side be damaged?
  4. Why does a creature moving through the barrier get a Reflex save for half damage, when a creature who has to suddenly jump out of the way gets a Reflex save to take no damage at all? Maybe this is backwards?
  5. Can a creature perform a melee attack through the barrier (such as with a longsword) without requiring a Reflex save? After all, the spell says the save is necessary when moving through the barrier. Or would that only apply to reach weapons?

These are just the major problems we had. Other more minor issues came up but I think if these are answered the others will be as well.

I'm currently going through and updating my "house rules" document. Your input is appreciated. :)

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I'm not sure the rules-as-written tag is going to work here, because I don't think this is really covered in the rules. There's a rules of the game web article, and I can't find much else. Since you also mention that you're talking about house rules, it might make sense to remove that tag. –  Tridus Mar 24 at 21:57
    
I would more like to regard his "house rules" as "rules compendium for controversial issues". –  Arle Camille Mar 24 at 22:43
    
Thanks to both of you. Yes, it's really "house rules to clarify things that aren't covered in RAW". But I've been surprised before because someone finds a RAW answer when I didn't think one existed, so I wanted to start there. I've removed that tag, however; I'm new here and you know better than I how it should work. :) –  Azhrei Mar 24 at 22:55
1  
@Azhrei If you really want RAW only answers, the tag is correct. :) But it's a bit weird to want RAW only answers while mentioning house rules at the same time. It's at your discretion which way you want it to go. I just wanted to raise a potential issue. :) –  Tridus Mar 24 at 23:07
    
Not going to post an answer because I don't have much to say - but in the group I DM I usually rule 'draw a straight line 20ft long / level - any square touched is now (on fire/covered in blades/etc).', and any creature occupying any of the affected squares takes the full spell effect. But of course that's just my house rule ^^ –  Commander Coriander Salamander Mar 27 at 17:31

1 Answer 1

up vote 2 down vote accepted

1. Radius Patterns follow Grid Lines

The ring version of Wall of Fire says this about its area of effect:

a ring of fire with a radius of up to 5 ft. per two levels

If you turn to the back of the DMG (p. 307) or look at this answer, you can see some patterns for radius spread effects. All of them spread along square edges. They're a ring of squares, rather than a circle. In this pattern, a square can't be half in or half out, because it only travels along the lines.

(This is actually also true for line effects, as PHB p. 176 shows. If a line goes through any part of a square, the entire square is affected. For something like Wall of Fire, an entire square has to be in one state or the other to follow with this, but if you place it along the grid lines it's fairly easy to do that.)

2. Yes

As you noted, Blade Barrier doesn't give a thickness or anything to imply that it's more than a curtain of blades (in fact, it says its a curtain of blades). You can treat it the same way as Wall of Fire: draw it on the grid lines. (The rules for area of spells mention that you should draw it starting from a grid intersection, so you can follow the grid lines.)

Regardless of the shape of the area, you select the point where the spell originates, but otherwise you don’t control which creatures or objects the spell affects. The point of origin of a spell is always a grid intersection. When determining whether a given creature is within the area of a spell, count out the distance from the point of origin in squares just as you do when moving a character or when determining the range for a ranged attack. The only difference is that instead of counting from the center of one square to the center of the next, you count from intersection to intersection.

This also makes the cover part of it easy to handle: anything on one side of that grid line attacking the other side has to deal with the cover portion of the spell.

3. Creature Size Matters

If you're placing it on grid lines (as you should), then yes, you can't actually place it on top of a medium creature. Large (or bigger) creatures take up multiple squares and you could have the barrier cast such that it appears in the middle of one. That makes this part of the spell make sense:

If you evoke the barrier so that it appears where creatures are, each creature takes damage as if passing through the wall. Each such creature can avoid the wall (ending up on the side of its choice) and thus take no damage by making a successful Reflex save.

Medium or smaller creatures wouldn't be damaged, as the barrier is not "where the creatures are".

4. Because the spell says so

This point has no answer, except because the spell says so. :)

I'd speculate that the game designers did that with the reasoning that as the spell comes into existence, someone can react and get out of the way before it can fully attack. Whereas once it's already there, someone has to go through it. But I don't know of any rules, anywhere, that explain why they did it this way.

5. Yes, you can attack

Blade Barrier says this:

Any creature passing through the wall takes 1d6 points of damage per caster level (maximum 15d6), with a Reflex save for half damage.

By a strict reading, passing through the wall is movement. By that reading: yes, you can attack through it without taking damage. I'm not aware of any rules clarification on that point.

Anybody attacking through it does have to deal with cover:

A blade barrier provides cover (+4 bonus to AC, +2 bonus on Reflex saves) against attacks made through it.

share|improve this answer
    
Thanks for your reply. I looked at that linked answer first. Your answer for #2 does make #3 seem obvious. :) But #1 (and #3) clash with the 3.5e PHB, page 176, which shows a line cutting through squares with a description that reads, "A line describes some kinds of attacks (usually magical). It affects creatures or characters in a straight line away from the spellcaster's square [...] All squares through which the line passes or touches are affected [...]". I can't find any support for the concept that a line effect bends around corners so that entire squares are affected. –  Azhrei Mar 24 at 22:52
    
@Azhrei If you look at the diagram on PHB p. 176, the line affects entire squares. Even when it crosses an intersection or is only partially in one square, the entire square is affected. That means in one spot it manages to hit two side-by-side squares. There's no squares that are only partially hit, which I think is the point. I'll update to clarify it. –  Tridus Mar 24 at 23:02
    
Ah, I see your point. The distance of the line is covered by the "straight line" but the jagged line is the result of which squares are touched by that straight line. But what about the thickness of the wall or the barrier? If a barrier is created, does it occupy the entire square? Or is it a jagged line that follows the grid? –  Azhrei Mar 24 at 23:07
    
@Azhrei I've covered the thickness as best I can above. There's no RAW answer, except that a "curtain of blades" doesn't sound very thick and wouldn't occupy a square. This rules of the game article says the same thing. Those articles aren't typically useful in RAW tagged questions, but I'll mention it now since you removed the tag. For these kinds of thin barriers, I've always ruled them as following the grid, because the mechanics of them just work so much more smoothly that way. –  Tridus Mar 24 at 23:13
    
There is actually a RAW answer for barriers: area effects are always placed on the grid intersections (straight or diagonally). See Area in the Spell Descriptions section of the SRD. –  SevenSidedDie Mar 25 at 15:55

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