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Triggered by the question about a fireball in a box, I was curious as to what the specific order of operations is with respect to spell resolution.

I have been unable to find any information which states that a spell's area of effect is limited to it's initial casting point.

Example scenarios:

A fireball is inside a wooden box. Does it destroy the box and expand to 20'?

If not, then consider that if the fireball was inside a cotton shirt. Or a bubble.

An additional example would be to swap fireball out for something like Lightning Bolt. Would hanging out behind a curtain suddenly make the spell utterly useless against you if area of effect is determined before the damage is?

I feel that spells area of effect and damage are not exclusive of each other, and that objects which are destroyed don't successfully impede the spell.

Is this accurate?

Additional information I've found which created the problem:

PHB pg. 196 under Cover

A target with total cover can’t be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

PHB pg. 204 under Area of Effect

A spell’s effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area o f effect, that location isn’t included in the spell’s area. To block one of these imaginary lines, an obstruction must provide total cover, as explained in chapter 9.

These two rules indicate that something as simple as a 5' X 5' roll of linen, so long as it is granting you full cover, is capable of withstanding any AOE spell and excluding you from the area. What's more, the wording seems to contradict each other. Spell AOE says it doesn't affect areas under total cover. Total cover states spell AOE can reach you. That's...... not helpful.

So I've attached an image which shows a Caster (denoted with a C) launching a lightning bolt through a room with 5 targets to help illustrate an example of this issue.

Casting a spell with obstacles in the way

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    \$\begingroup\$ I don't think the wording of the quotes contradict each other. 1st quote, you can't directly target someone under total cover in relation to you, but you can still "hit" someone under total cover within the area of a spell. For example, enemy totally covered behind a wagon. Can't target it with Magic Missile. But you can aim the point of origin of a Fireball past the side of the wagon, thus hitting a wide area including enemy, wagon, everything behind it. 2nd quote validates this example. To be hidden from the Fireball, enemy would need total cover in relation to the Fireball point of origin. \$\endgroup\$ – RafaelLVX Jul 21 '16 at 20:04
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Like so many things in D&D 5e, this is left as a call to be made by “the agent on the ground” rather than “headquarters” — that is, it's up to the DM who can actually see the situation involved and made the best judgement call and ruling. The fundamental rules for the spell are simple enough to work in most situations, but don't bother to attempt the Sisyphean task of pre-specifying how every possible corner case should work.

Is a cotton shirt a barrier? Is a soap bubble? Is a wooden box? Should there be time-based alterations to the area of effect? Some of these things are beyond the remit of the rules, and are explicitly DM calls to make. (Some of these things are not — a cotton shirt, linen curtain, or soap bubble are not reasonable to class as “obstructions” like creatures or walls are, and wouldn't stop a fireball nor lightning bolt. See PHB pages 196 and 204.)

This DM definitely wouldn't count soap bubbles, flimsy curtains, or cotton shirts as relevant to the area of effect. For the less obvious parts, I'd also say that a wooden box couldn't fully contain it. But I would have so many questions about the box before I could say what did happen. How big is the box? How thick the wood? Who made it and how well or crudely? What kind of wood is it? How did the fireball even get in there? I wouldn't be using these in some kind of formula, mind — they'd all feed into my sense of the situation's state of affairs and colour the exact nature of the events that follow the conflagration.

Similarly for that wooden wall in the path of the lightning bolt — the default is that it stops the spell's effect, but if there are extenuating circumstances then they are, well, extenuating. If it's not obviously a durable obstacle in the actual moment, it would require thought applied to the specific details of the case to make a ruling on what exactly does happen instead of the default.

Which is, for better or worse, how 5e is written to work.

In general though, yes, a spell's area is determined and set before damage is applied. (This just follows from a face-value reading of “Areas of Effect”, PHB, page 204.) Usually that's so obviously the case that it would seem odd to even need to assert it in the first place. The rare exceptions where that's not obviously how it should work — like fireballs inside too-small burnable boxes — get thrown to the the DM, to handle as an unforeseen/unforeseeable exceptions.

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The spell description for fireball says:

A bright streak flashes from your pointing finger to a point you choose within range and then blossoms with a low roar into an explosion of flame.... The fire spreads around corners. It ignites flammable objects in the area that aren't being worn or carried.

So the box couldn't be sealing the spell in so there must be corners for the fire to spread around, so it will fill the 20'radius with no problem, doesn't matter if the box is destroyed or not.

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    \$\begingroup\$ Why the down votes on this answer? \$\endgroup\$ – KorvinStarmast Jul 19 '16 at 15:05
  • \$\begingroup\$ Because it is bad and misleading and wrong, in various ways. \$\endgroup\$ – GreySage Jul 19 '16 at 16:12
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    \$\begingroup\$ Care to elaborate? Can't improve an answer without know why people think it's wrong. \$\endgroup\$ – yinzanat Jul 19 '16 at 18:33
  • \$\begingroup\$ I wasn't a down-vote, but (1) attribute your quote with a page number, and (2) format it as a quotation. (3) The reasoning why a box couldn't seal a fireball in is not clear - why are there necessarily corners to spread around again? (4) Spell names are capitalized, and probably bolded/italicized - because they are not just words they are special terms. \$\endgroup\$ – Tim Grant Jul 20 '16 at 6:19

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