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In D&D 4e can you target an area spell on an area that is not visible.

For example a rogue uses an ability to give total concealment over an area, can a wizard cast fireball centered in that area of darkness?

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up vote 9 down vote accepted

D&D Rules Compendium

p106 "Line Of Sight: Typically the user of a power doesn't have to be able to see the power's targets ... "

p107-8 "Line of Effect: When there is a clear line from one point to another in an encounter, there is line of effect ... Fog, darkness, and smiliarly obscured squares block vision, but they don't block line of effect. If Albanon the Wizard hurls a fireball into a pitch-black room, he doesn't have to see the creatures in the room for the fire to hit them."

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Ergo, unless the power explicitly requires line of sight, or uses language such as "one target you can see", you may target squares you cannot see with powers. –  Simon Withers Sep 28 '10 at 12:48
    
You must have line of effect though. –  Pat Ludwig Sep 28 '10 at 14:25
    
I haven't read the RC myself, but the wording above seems to imply that having line of sight = having line of effect. This is not always true, in cases such as looking through a window or other transparent solid object. In those situations, you have line of sight, but normally don't have line of effect. –  Iszi Sep 29 '10 at 13:59
    
@Iszi. Yes. The excerpts do seem to imply that, but the context makes it clear that line of sight and line of effect are largely independent factors. –  Viktor Haag Sep 30 '10 at 15:05
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It should depend on what is listed as the spell's target. If the spell can be cast on a location rather than a character/creature, then it can be done. That should hold true for fireball as well. It can be cast on a location.

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This is not strictly the case according to the new RC: you very specifically do not necessarily have to have line of sight to a target; what you need is line of effect, and darkness doesn't block that. –  Viktor Haag Sep 28 '10 at 12:32
    
@Viktor, I was thinking that I said that you can cast a fireball into darkness :) –  edgerunner Sep 29 '10 at 21:25
    
You're almost certainly correct. Apologies. 8/ –  Viktor Haag Sep 30 '10 at 14:38
    
Area attacks are always cast on a location, the Origin Square. The "Target" definition in an Area attack just clarifies what conditions something in the area must meet in order to be affected by the spell. –  Iszi Sep 30 '10 at 16:56
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Page 271 of the Player's Handbook, area attacks:

[...] You need line of effect from a square in your space to the origin square. For a target to be affected by an area attack, there needs to be line of effect from the origin square to the target. You don't have to be able to see the origin square or the target, and concealment between the origin square and the target doesn't apply.

In other words, if the targeted square is blocked by something like fog, smoke, or the like you can still target it. If it's blocked by a solid wall, you can't.

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Yes, the wizard can center the fireball on the center of the darkness

  • Assumption: The wizard has line of effect to the center of the darkness.
  • Note: The location of the rogue is always known unless the rogue is hidden.

Area Powers are defined on page 104 of the Rules Compendium. There are only two conditions for the Origin Square (center of the effect)

  1. The Origin square must be within the range of your area power
  2. You must have line of effect to the origin square

If the rogue is hidden (as he should be!) and you don't know his exact location, then the DM must determine if the rogue is a valid target of the burst.

  • The rogue must be within the burst (3 for fireball)
  • Must conform to the target line of the power (each creature in burst for fireball)
  • There must be line of effect from the origin square to the rogue.
  • Line of Effect from the wizard to the rogue is not needed.

If those conditions are met, expect to hear a howl of pain from the rogue!

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While the rules allow the targeting (well established on PHB p.271), one would, as a good GM, be well within rights to ask how the character knows to target a specific square.

If the character would not know of the targeted area (such as a darkness cast in an unlight room outside the light radius of the character to hide a thief waiting to surprise the PC's), such a fireball might be disallowed, or might require a Notice check to hear something and give reason to target.

On the other hand, if the thief triggered the concealment while in line of sight, an immediate fireball at his specific square would be perfectly reasonable as the wizard's next action.

If the Character shouldn't know, the player should not be permitted to act as if the character does know.

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Well the GM was the one with the fireball, but yes, it was perfectly reasonable. That's for pointing that out! –  C. Ross Sep 28 '10 at 22:55
    
The GM should be treating all NPC attacks that way, IMO... "Does this character know?" If not, do something else. –  aramis Sep 28 '10 at 23:03
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They popped darkness in a narrow hallway right in front of the mage in question; it seemed pretty reasonable as a next act to me as well :) –  rjstreet Sep 29 '10 at 0:17
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Ah-yup.., sounds reasonable to me, too. –  aramis Sep 29 '10 at 4:53
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I think the below excerpt from the "Area Attack" entry in the Compendium covers it all. The last line specifically says that you don't need line of sight to the Origin Square or target creatures, only line of effect. The document referenced in the Compendium is the Player's Handbook.

Origin Square: You choose a square within an area attack’s range as the attack’s origin square, which is where you center or start the area of effect. You need line of effect from a square in your space to the origin square. For a target to be affected by an area attack, there needs to be line of effect from the origin square to the target. You don’t have to be able to see the origin square or the target, and concealment between the origin square and the target doesn’t apply.

Consider the following: There's a dark room in front of you. It's too dark to see inside, but you just watched your enemy run into the room. A hand grenade on your belt is your only usable weapon at this time.

  • What do you do? You estimate how far you need to roll/throw the grenade to reach the center of the room, and roll/throw the grenade for that distance.
  • What happens to the grenade? It travels into the room for as far as you've rolled/thrown it, provided there are no obstructions, and detonates.
  • What happens to the enemy, if he was in the room when the grenade detonated? To quote Major Payne: "If he's still in there, he ain't happy!"
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