When does a Magical Sensor come into being?

[bret]

Here is a question I've not seen asked before.

When does the magical sensor (such as from the Scry spell) come into being?

The scry spell takes an hour to cast and lasts for minutes. What isn't clear is if the person being scryed on would have a chance to detect the sensor before the Scryer was able to use the sensor.

Simple example: Wizard casts Detect Scrying. If someone scrys within 120 ft of the wizard, would he have a chance to shout a warning before the person scrying would detect it? Would he have a chance of Dispelling the sensor before it fully formed?


More insidious use: Theives guild (being naturally paranoid because people ARE out to get them) want to quickly switch to misinformation if someone scrys on their meeting. Basically the whole meeting now becomes a bluff to feed the 'unwelcome visitor' misinformation. This is a planned for occurance, part of the standard security plan.

Do they have enough time to switch conversations before the scryer would notice that a topic change had occurred, or would they have to slowly shift in order to avoid raising suspicion on the part of the scryer?


Having any indication of the sensor before data gets make would have a huge effect on security procedures.

 

[kreynolds]

When does the magical sensor (such as from the Scry spell) come into being?

When the spell is cast.

The scry spell takes an hour to cast

Exactly, which means the spell doesn't take effect until the casting is completed.

 

[Lucius Foxhound]

I'm with KReynolds on this one. From the SRD:

Casting Time

... A spell that takes 1 minute to cast comes into effect just before the character's turn 1 minute later (and for each of those 10 rounds, the character is casting a spell as a full-round action).

 

[bret]

I got curious enough that I reread the magic chapter and a few of the spells in question, looking for clues.

I think the sensor must exist before any information can be sent back from it. At very least, using a Ready action to Dispel a Magical Sensor should be effective.

I couldn't find anything that told me exactly when it does appear.

Lets look at the rules with respect to the Clairaudience/Clairvoyance (PHB 184) and Scrying (PHB 247-8) spells.

PHB 148 Casting time

You must make all pertinent decisions about a spell (range, target, area, effect, version, etc) when you begin casting. For example, when casting a summon monster spell, you need to decide where you want the monster to appear.

This means you have to select where the invisible sensor for a Clairaudience/Clairvoyance spell will appear before you start casting it. The sensor is the effect which must be placed.

PHB 150 The spell's Result

Once you know which creatures (or objects or areas) are effected, and whether those creatures have made successful saving throws (if any) you can apply whatever results a spell entails.

The results from Divination spells is information. You can't start sending any information back from the magical sensor until the spell has completed. This comes after any chances for counterspelling, saving throws, spell resistence, etc. I take this to mean that the Magical Sensor has to have been created before this point, otherwise there would be nothing to feed the results back with.

If someone can see a person casting a Clairaudience/Clairvoyance spell, they could counterspell it. Seems the same would apply to Scry. There has some none zero time between when the spell is completely cast and the spell results occur. That is the time during which you would normally be doing saving throws, spell resistence checks, etc.

Spells are cast in the same way. It does not matter if the spell in question allows a saving throw or spell resistence, the time would be there for these things to happen it is just that some spells are immune to those type of defenses.

So it seems to me that there must be some non-zero time between the casting of Clairaudience and when you first see something through the sensor that the magical sensor was in place.

So I ask again, when does the Magical Sensor appear for Scry or similiar spells?

 

[Lucius Foxhound]

Check it out: "A spell that takes 1 minute to cast ... comes into effect just before the character's turn 1 minute later."

There's your answer. JUST BEFORE your turn. I'd rule it was a fraction of second before your turn.

Just because YOU need to make all pertinent decisions about a spell when you begin casting, doesn't mean anyone else knows anything. Unless someone sees you casting the spell, they don't have any information that a spell is about to be cast in their location.