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I have just recently picked up the D&D 4th edition rulebooks and I’m a little confused about certain aspects of the rules regarding how to combine the rules regarding line of sight, cover, and area of effect.

Let's imagine the following situation:

battle map

Three PC ("W", "F" and "R") shook off the goblins that were chasing them. The Goblins are looking for them in the room North of where they took refuge. One of the goblins opens the door to where two of the player characters are hidding. The Wizard PC, who went ahead to explore the room to the right, overhears the goblin opening the door, encourages his mates to take cover and throws a Fireball spell in the middle of that room (C-9).

My understanding of the rules is that:

  1. The wizard in H-9 can legally cast Fireball through the doorway to C-9, whether he has "line of sight" to where it's going to land or not, as long as the square used as a point of origin for the spell is located less than 20 squares away from the caster.
  2. Being an “area burst 3” spell, the effects of the Fireball expand 3 squares from its point of origin to every direction (effectively creating a 7*7 square) .

In the example above, the spell’s area of effect is enough to fill the entire room and spill to F-9(not drawn on the map above, my mistake). As a consequence, 3 characters are located within the Fireball’s burst radius: - the goblin in C-6, against which the wizard ought to roll a dice against and who does not benefit from cover of any kind, but also, - the rogue and the Fighter who are literally caught in friendly fire: they aren’t entirely safe from spell’s effect but at least get some cover from the pillar in B-10. The action is resolved by the wizard having to roll against them but has to take the cover into account (-4 in that case).

  1. If the above is correct, would the situation be different for either of the PCs if she were in A-11, for instance? Wouldn’t her location relative to the pillar in B-11 “shield” her from the effects of the burst? She would then be safe, in addition to being “covered”?
  2. Am I correct in assuming that if the wizard was standing in F-9 instead and casting the Fireball in C-9, he would not get any cover from the open doorway between E-9 and F-9 and would risk being caught in his own spell?
  3. Am I correct in assuming that the wizard could legally throw a Fireball in C-9 if he were located in G-6, for instance (that would be 9 squares away)? My understanding is that area burst spells can pretty much be “hail Maries” as long as the caster is within range, yet I assume that wouldn’t work if the caster had no notion of what the room in the lower left corner looks like at all. Is that correct?

Thank you very much for your help.

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1) The wizard needs line of effect.

I will have to double-check the rules on whether line of sight is needed (I think it's not), but line of effect is definitely needed. To clarify the difference between the two, a glass wall in E6-E11 would block line of effect but not line of sight, while a smoke cloud in the same squares would block line of sight but not line of effect.

2) The fireball fills the room (almost, see 3) as well as F9.

Any square that is at most 3 squares away from the point of origin and for which an unblocked line can be drawn from at least one corner of the origin square to at least one corner of the target square is included. This includes the goblin, the fighter, and the rogue, though as mentioned the fighter and rogue have cover (which is -2 not -4, incidentally).

3) A11 is not included in the burst.

There are no corners of the point of origin square (C9) that can draw an uninterrupted line to a corner of A11. Therefore the point of origin does not have line of effect to A11, so A11 is not included in the burst.

4) The wizard would be included if he was in F9.

As mentioned, F9 is included in the blast because it is both in range and in line of effect. I'm pretty sure Fireball targets all creatures in the burst, and the wizard is (presumably) a creature, so he would be affected by his own fireball. Note that including yourself in burst/blast attacks is something you would occasionally do on purpose, such as when rocket jumping or in a dragonborn rebreather build.

5) No, the wizard could not target C9 from G6.

As mentioned above, regardless of whether or not you have line of sight you always need line of effect to your target. Area burst spells allow a little bit of indirection; you need line of effect to the origin square, and the origin square needs line of effect to the target.

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    \$\begingroup\$ Are you sure about #3? Since the diagram I found indicated that the line from the top left of C9 to the top left of A11 would be a valid line (just touching the corner of the pillar wouldn't block it). You are correct about not needing LOS though, only LOE is needed. \$\endgroup\$ – diego Oct 27 '16 at 14:16
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    \$\begingroup\$ @diego It will have to wait about 10 hours until I get home to check the RC, but I'm 90% sure that touching the corner of the pillar does block it. It's conceptually related to not being able to move diagonally when it would cut through a solid corner (which is why F8 & F10 are not included). \$\endgroup\$ – Oblivious Sage Oct 27 '16 at 14:34
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When making an area attack your chosen origin square must be within "line of effect" from you not "line of sight". [DDI] And when determining cover for area attacks you do it the same way you would for ranged attacks except the origin of the area effect is used as the attacker's square, and creatures don't provide cover. [DDI] This means that you pick a corner of the origin square and attempt to draw lines to all the corners of the square the target is in, if all four lines are unblocked they have no cover, if 2 or 3 lines are unblocked they have cover, if only 1 line if unblocked they have superior cover, and if all the lines are blocked they can't be targeted. [DDI]

So currently both the fighter and rogue have cover since there are 2 unblocked lines you can draw from a corner of the origin square to the corners of their squares and the goblin has no cover.

If a PC were in A11 there would be 1 unblocked line between a corner of the origin and their square so they would have superior cover.

If the Wizard were in F9 they would be in the area of effect and have no cover so they would get hit by their own fireball.

If the Wizard were in G6 he would not be able to use fireball on C9 because there are wall blocking their line of effect.

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