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:
- 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?
- 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?
- 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?
- 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?
- 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. 🙂
1. Radius Patterns follow Grid Lines
The ring version of Wall of Fire says this about its area of effect:
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.)
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.)
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:
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 rules-as-written 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:
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: