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I'm a little fuzzy on how spell traps work with being triggered, so let me set up an example:

A Fireball trap. It hits a 20ft radius, all targets in that area. Triggered via proximity of the Alarm spell, which based both on the spell description and the spell effect is also the 20ft radius area. I assume when placing a trap of this nature, you still pick a square or intersection depending on the case, which would be the same point for both spells, and in this case have the same circle of effect.

So does the fireball go off when the first person steps within that 20ft radius circle? If so, then assuming the party is moving one at a time in a round because the players take turns and also assuming it wasn't detected, then only the first person would take any damage. Am I understanding this correctly? If so that seems like a waste.

Please help me understand how this is meant to work. To me it would make more sense to have a smaller radius trigger to get the whole party into the area before the trap detonates in the case of an AoE effect like fireball. I feel like I am either missing something or not understanding the system correctly.

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  • \$\begingroup\$ Thanks for breaking that up. Wrote the question at work and didn't have time to format nicely. \$\endgroup\$ – Omnitus Sep 19 '15 at 17:54
  • \$\begingroup\$ Consider the fact that out of combat people aren't moving turn-by-turn. DM usually asks the group to select a formation in which they advance through the dungeon (or to describe their real time movement in some other way). So when the first PC enters the warded area, others shouldn't necessarily be out-of-range. \$\endgroup\$ – annoying imp Sep 20 '15 at 7:09
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A trap's creator can elect to reduce the spell alarm's area of effect when the spell is used a trap's trigger

While the spell alarm usually has a 20-ft.-radius area of effect, when used for the trigger of a trap, this area can be reduced to far less than that.

For example, the section on Proximity Trigger says, in part, that

This trigger activates the trap when a creature approaches within a certain distance of it. A proximity trigger differs from a location trigger in that the creature need not be standing in a particular square. Creatures that are flying can spring a trap with a proximity trigger but not one with a location trigger. Mechanical proximity triggers are extremely sensitive to the slightest change in the air. This makes them useful only in places such as crypts, where the air is unusually still.

The proximity trigger used most often for magic device traps is the alarm spell. Unlike when the spell is cast, an alarm spell used as a trigger can have an area that's no larger than the area the trap is meant to protect.

Emphasis mine. That phrasing is really weird, but it means, in essence, if a trap's protecting a 10-ft. square, the alarm spell's area can't exceed the 10-ft. square being protected. Further, the section on Touch Triggers says

A touch trigger, which springs the trap when touched, is one of the simplest kinds of trigger to construct. This trigger may be physically attached to the part of the mechanism that deals the damage or it may not. You can make a magic touch trigger by adding alarm to the trap and reducing the area of the effect to cover only the trigger spot.

Emphasis mine, and that emphasized text strongly implies that in trap construction, the spell alarm can be made to cover whatever size the trap creator wants up to the limits of the spell alarm.

Combined, this means the spell alarm can be used as a trap's trigger to discharge effects when upon a creature approaching within 20 ft., 15 ft., 10 ft., 5 ft., or 0 ft., or when a specific spot is touched. Further, the metamagic feat Widen Spell can be applied to the spell alarm if the trap's creator needs to cover a much larger area than the alarm spell normally allows.

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