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When you cast a spell with “effect” entry, do you have to have line of effect to every portion of the effect to be created?

Some examples of where it can be relevant:

  • Casting Summon Monster to conjure something really big. Depending on the answer, caster may be able to conjure say colossal creature to a place where he can't place medium one (an edge of a cliff spell's range doesn't cover). Or the size may instead limit the range as you presumably have only 15' wide 10' high area into which you have line of effect at the end of spell's range (so something bigger than large simply won't fit in).
  • This may be (and may not be) even more complicated with Wall of X kind of spells. Do you need line of effect to every square the wall is to appear in? If the wall is assembled from 10' cubes (or 10' squares with some thickness), do you only need line of effect to centers of those? Do you designate a single point of origin within effect's volume somehow?
  • Sphere or globe effects. This is of course true those effects are partially blocked by thick solid masses (like the ground under a character's feet), but what if you have a pillar or column about 6' in diameter 6' high right next to you? Would such an obstacle prevent Prismatic Sphere to close around you leaving the gap in your defenses?
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A needs line of effect to every space into which a spell will create an effect

The Player's Handbook on Line of Effect, in part, says, "You must have a clear line of effect… to any space in which you wish to create an effect" (176). Further, on Effect says

You must designate the location where these things are to appear, either by seeing it or defining it (for example, "The insect plague [effect] will appear 20 feet into the area of darkness that the nagas are hiding in"). Range determines how far away an effect can appear, but if the effect is mobile (a summoned monster, for instance), it can move regardless of the spell’s range." (175 and link mine)

Further, special rules apply to effects brought forth by spells of the magic school conjuration:

A creature or object brought into being or transported to your location by a conjuration spell cannot appear inside another creature or object, nor can it appear floating in an empty space. It must arrive in an open location on a surface capable of supporting it. (172)

(It's a mystery to this reader why the printed text presents this information in exactly the opposite order that the reader needs it!)

A spell of the subschool summoning typically creates an effect, so the caster needs line of effect to the entire space into which the summoned creature or object will appear. Likewise, a caster needs line of effect to the entire space where a wall spell—like the 6th-level Sor/Wiz spell wall of iron [conj] (PH 299)—will see its effect created. Finally, a caster needs line of effect to the entire space where a sphere spell—like the 4th-level Sor/Wiz spell Otiluke's resilient sphere [evoc] (PH 258)—will see its effect created.

Finally, the Player's Handbook on Line of Effect also says, "An otherwise solid barrier with a hole of at least 1 square foot through it does not block a spell's line of effect" (176). A 6-ft. diameter pillar occupies the square it's in and some of the surrounding squares, so it's a barrier and would prevent the creation of effect spells in that the square it wholly occupies. However, in the surrounding squares—into which the pillar's diameter extends only 6" or so—there is much more than a square foot into which to put stuff, and it wouldn't block line of effect. (By the way, a pillar in D&D 3.5 is usually either slender—not occupying a square therefore not a barrier—or wide—occupying a square therefore a barrier.)

The problem with prismatic sphere et al.

Some sphere spells—like the 9th-level Sor/Wiz spell prismatic sphere [abjur] (PH 264)—are given Effect entries the DM should probably modify these spells.

That is, when such a spell tries to create an effect that's a sphere around a creature and that sphere effect would extend beyond the creature, such a sphere spell should create an effect that's a dome over a creature that's on a surface, the remainder of the spell's effect occluded by the surface. However, a spell that creates an effect that's a big enough sphere—like the prismatic sphere spell's 10-ft.-radius one—means that unless the caster somehow has line of effect to the space beneath the target the spell should, instead, technically fail as the caster doesn't have clear line of effect to those spaces because those spaces themselves aren't clear.

The Player's Handbook on Spell Failure says

If you ever try to cast a spell in conditions where the characteristics of the spell (range, area, or the like) cannot be made to conform, the casting fails and the spell is wasted. (171)

And a spell like prismatic sphere doesn't allow any choices when its effect is created. For example, the spell wall of iron has Effect: Iron wall whose area is up to one 5-ft. square/level, but a prismatic sphere spell has Effect: 10-ft.-radius sphere centered on you, without an up to and without any concessions for creating an effect at anything less than its maximum—and only—size.

In sum, by a strict reading, such a spell can affect only a creature that's flying, swimming, and the like. To avoid pointlessly penalizing such spells and having archmages looking like nitwits because they forgot this technicality, this DM recommends changing such spells on a case-by-case basis when appropriate, and, as always, the DM first informing players of this change.

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  • \$\begingroup\$ Why do you think half-sphere should fail as written? "You must have a clear line of effect… to any space in which you wish to create an effect" may mean 'You don't have LoE to that square? Effect doesn't manifest in this (and only this) square' not neccessarily 'You don't have LoE to one square among those an effect to appear in? You can't create an entire effect'. You can't summon half of a monster - that's true, but I can't see whay you can't create half of a wall (or sphere). Am I missing something? \$\endgroup\$ – annoying imp Mar 20 at 8:37
  • \$\begingroup\$ I mean making Prismatic Sphere an area will probably change how does this spell work. Area spells usually affect everything up to a listed radius, not just the border of an area. You probably may make it a spell with a specific kind of area, but you'll probably have to be carefull with the wording. \$\endgroup\$ – annoying imp Mar 20 at 8:42
  • \$\begingroup\$ @annoyingimp You're correct that making it an area spell is the wrong way to go, but I updated the answer to explain why a half-sphere effect can't technically be created. \$\endgroup\$ – Hey I Can Chan Mar 20 at 10:59
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The general rule (described in "Line of Effect" under "Aiming a Spell") is that line of effect from the caster is required to create an effect in a square. There is an exception (in the "Area" section) for bursts, spreads, and emanations: they require line of effect from the caster to the point of origin and from the point of origin to any affected square (but not necessarily from the caster to the square).

The first two examples in the question (summoning a creature and creating a wall) fall under the general case, and require line of effect from the caster to all affected squares. The final example (a spherical burst) falls under the specific exception, and requires line of effect from the point of origin to all affected squares (Prismatic Sphere in particular calls out in its description that half of the sphere will generally not be created due to the caster standing on a solid surface).

That said, in my personal experience, it's a quite common house rule to extend the special rules for bursts, emanations, and spreads to all spells. In these cases, the point of origin is generally either the center or an edge of the effect. I've not found this to be disruptive to any game I've played in, and have found that, subjectively, it improves enjoyment of the game more than it detracts from it.

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  • \$\begingroup\$ I should note that Prismatic Sphere isn't a burst. It is not area spell to begin with, and burst is a kind of an area not a kind of effect. \$\endgroup\$ – annoying imp Mar 20 at 8:53
  • \$\begingroup\$ I do not see that the final example talks about bursts. It talks about Sphere or globe effects. It can be a spread, and spreads do not need line of effect. \$\endgroup\$ – Ols Mar 20 at 13:52
  • \$\begingroup\$ I mean they don't need a line of effect from their point of origin to all affected squares. \$\endgroup\$ – Ols Mar 20 at 18:05

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