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I'm really not sure if that's any clearer.
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The rules on Aiming a Spell on Line of Effect explain how these spells' effects interact rather than the spells themselves

Imagine the wall of force effect as an actual wall

The wall of force spell's description saysMost walls are solid barriers, "A wall of force creates an invisible wall of pure force.… Breath weapons and spells cannot pass through athat create effects often can't create their effects wall of forcethrough [effect] in either directionwalls." Thus the spell wall of force creates an effect that is, essentiallySpell Failure says, "If you ever try to cast a solid barrier that blocks linespell in conditions where the characteristics of effectthe spell cannot be made to conform, the casting fails and "[y]ou must have a clear line of effect to… any space in which you wish to create an effect" (see Aiming a Spell on Line of Effect)the spell is wasted."

And theThe spell blade barrier creates such an effect. The caster that, in this case, is "a ringed wall of whirling blades with a radius of up to 5 ft. per two levels [that's] 20 ft. high." Casting a blade barrier spell so that its effect overlaps with a low wall either causes the spell to fail blade barrier doesor, more generously, causes the notblade barrier have line of effect to the spaceswrap beyondover the low wall… wall of forcenot effectgo (even thoughthrough the wall.

That is, the blade barrier's caster may have effect can't be shaped—in this case only its radius can be picked by the caster—so part of the line of sightblade barrier to them), so the castereffect must be can'tthrough designate those spaces beyondthe wall. Yet the wall of forceblade barrier effect as beingmust stop at the subjectwall and be on both sides of it. Thus the blade barrier effect is no longer contiguous, a necessity for the effect's successful creation. AThe GM may have the spell simply fail ifrule that the caster triedblade barrier effect conforms to designate such spacesthe wall, wrapping around or over it ("If you ever try to castyet not through it), but a spell in conditions whereGM may instead rule that—because the characteristics of the spell cannotcan't be made to conform, to the wall—the casting the fails, and the spell is wasted"; see Spell Failure)spell's wasted. In either case, butthough, the wall (or a more generous GM—including this one—would havewall of force effect) would remain unharmed by the blade barrier effect.

Essentially, accordingwhat's being tried here is like trying to summon a Huge (tall) creature into a 15-ft.-by-15-ft. space that's split by a 10-ft.-high-yet-paper-thin wall: from several vantages the caster's desires, do its bestcaster may have line of effect to accommodateall of the squares individually wall of force effect yet leave(see Aiming a Spell on Line of Effect), but the spell's wall of forceoverall effect unharmedcan't be made to conform to the space into which the caster wants to put it. Trying to do that typically just ends up with the spell failing.

The line of effect rules prevent havingThis is different from an area spell that's a burst or spread like the blade barrierfireball effect intersect with, go beyond, andspell (under the house rules describedsee Aiming a Spell on Area) damage periodically. The caster of the wall of forcefireball effectspell picks a grid intersection as the spell effect's point of origin and counts squares from that point of origin to determine what's caught in the spell's area. AllowingThis allows a fireball spell that's cast up in the air to affect stuff on blade barrierboth effect—and lowersides of a low-level and other more common spells that create effects likelying wall… wall of fire—toand the wall.

A caster doesn't need a ignorefireball linespell's area to be clear of effect createsobstructions for it to be used—that's handled by the burst mechanics—, but a dangerous precedentspell that sees sealed castles becoming blenders, ovens, and worsecreates an effect over an area requires more delicacy in its grid placement than a burst.

The rules on Aiming a Spell on Line of Effect explain how these spells' effects interact rather than the spells themselves

The wall of force spell's description says, "A wall of force creates an invisible wall of pure force.… Breath weapons and spells cannot pass through a wall of force [effect] in either direction." Thus the spell wall of force creates an effect that is, essentially, a solid barrier that blocks line of effect, and "[y]ou must have a clear line of effect to… any space in which you wish to create an effect" (see Aiming a Spell on Line of Effect).

And the spell blade barrier creates such an effect. The caster of the spell blade barrier does not have line of effect to the spaces beyond the wall of force effect (even though the blade barrier's caster may have line of sight to them), so the caster can't designate those spaces beyond the wall of force effect as being the subject of the blade barrier effect. A GM may have the spell simply fail if the caster tried to designate such spaces ("If you ever try to cast a spell in conditions where the characteristics of the spell cannot be made to conform, the casting fails and the spell is wasted"; see Spell Failure), but a more generous GM—including this one—would have the blade barrier effect, according to the caster's desires, do its best to accommodate the wall of force effect yet leave the wall of force effect unharmed.

The line of effect rules prevent having the blade barrier effect intersect with, go beyond, and (under the house rules described) damage periodically the wall of force effect. Allowing the blade barrier effect—and lower-level and other more common spells that create effects like wall of fire—to ignore line of effect creates a dangerous precedent that sees sealed castles becoming blenders, ovens, and worse.

Imagine the wall of force effect as an actual wall

Most walls are solid barriers, and spells that create effects often can't create their effects through walls. Spell Failure says, "If you ever try to cast a spell in conditions where the characteristics of the spell cannot be made to conform, the casting fails and the spell is wasted."

The spell blade barrier creates an effect that, in this case, is "a ringed wall of whirling blades with a radius of up to 5 ft. per two levels [that's] 20 ft. high." Casting a blade barrier spell so that its effect overlaps with a low wall either causes the spell to fail or, more generously, causes the blade barrier effect to wrap over the low wall… not go through the wall.

That is, the blade barrier effect can't be shaped—in this case only its radius can be picked by the caster—so part of the blade barrier effect must be through the wall. Yet the blade barrier effect must stop at the wall and be on both sides of it. Thus the blade barrier effect is no longer contiguous, a necessity for the effect's successful creation. The GM may rule that the blade barrier effect conforms to the wall, wrapping around or over it (yet not through it), but a GM may instead rule that—because the characteristics of the spell can't be made to conform to the wall—the casting the fails, and the spell's wasted. In either case, though, the wall (or a wall of force effect) would remain unharmed by the blade barrier effect.

Essentially, what's being tried here is like trying to summon a Huge (tall) creature into a 15-ft.-by-15-ft. space that's split by a 10-ft.-high-yet-paper-thin wall: from several vantages the caster may have line of effect to all of the squares individually (see Aiming a Spell on Line of Effect), but the spell's overall effect can't be made to conform to the space into which the caster wants to put it. Trying to do that typically just ends up with the spell failing.

This is different from an area spell that's a burst or spread like the fireball spell (see Aiming a Spell on Area). The caster of the fireball spell picks a grid intersection as the spell effect's point of origin and counts squares from that point of origin to determine what's caught in the spell's area. This allows a fireball spell that's cast up in the air to affect stuff on both sides of a low-lying wall… and the wall.

A caster doesn't need a fireball spell's area to be clear of obstructions for it to be used—that's handled by the burst mechanics—, but a spell that creates an effect over an area requires more delicacy in its grid placement than a burst.

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Hey I Can Chan
Hey I Can Chan
  • 192.7k
  • 18
  • 362
  • 876

The rules on Aiming a Spell on Line of Effect explain how these spells' effects interact rather than the spells themselves

The wall of force spell's description says, "A wall of force creates an invisible wall of pure force.… Breath weapons and spells cannot pass through a wall of force [effect] in either direction." Thus the spell wall of force creates an effect that is, essentially, a solid barrier that blocks line of effect, and "[y]ou must have a clear line of effect to… any space in which you wish to create an effect" (see Aiming a Spell on Line of Effect).

And the spell blade barrier creates such an effect. The caster of the spell blade barrier does not have line of effect to the spaces beyond the wall of force effect (even though the blade barrier's caster may have line of sight to them), so the caster can't designate those spaces beyond the wall of force effect as being the subject of the blade barrier effect. A GM may have the spell simply fail if the caster tried to designate such spaces ("If you ever try to cast a spell in conditions where the characteristics of the spell cannot be made to conform, the casting fails and the spell is wasted"; see Spell Failure), but a more generous GM—including this one—would have the blade barrier effect, according to the caster's desires, do its best to accommodate the wall of force effect yet leave the wall of force effect unharmed.

The line of effect rules prevent having the blade barrier effect intersect with, go beyond, and (under the house rules described) damage periodically the wall of force effect. Allowing the blade barrier effect—and lower-level and other more common spells that create effects like wall of fire—to ignore line of effect creates a dangerous precedent that sees sealed castles becoming blenders, ovens, and worse.