mechanics for disrupting spells

[taliesin15]

I'm having trouble as a DM ascertaining the mechanics for disrupting spells in many kinds of situations. Use of Spellcraft makes sense to me. But it seems like the casting time for most spells is practically instantaneous, so that the about the only way I can see a character purposefully disrupting the spell casting of an opposing character is if the former has readied an action to attack the opposed spellcaster once s/he begins to cast the spell.

One example: let's say the setting is an urban setting, a small city at dusk. An mid level (let's say 7th/7th) Rogue/Wizard Elf has been spying on a rich gem merchant for a few days, watching his place of residence, where he goes, how many bodyguards he usually has with him, which tavern he goes to, where his clients lives. Normally the Elf watches in shadows using his Hide skill. Now here's the scenario: while Hiding in an alley, some 50' away from the main thoroughfare where the merchant and bodyguards (let's say 6 5th level Human Fighters or Warriors) is passing. The Elf has a clear line of sight for ten seconds. He casts Detect Thoughts.

Assuming the merchant for bodyguards doesn't have any kind of magic item that picks up on spells being cast or magical thought intrusion protection/detection of any sort, what would the opposing checks be?

My thought is the merchant & entourage would be allowed a Spot check vs. the Elf's Hide skill. In this case, the Elf usually would not be Spotted. Other than that, there's the Will Save vs. the spell. It doesn't seem like there's any other way the merchant & entourage could tell that their thoughts are being detected.

Readied actions seem to be the only way to disrupt an opposing spellcaster. Especially if the spellcaster has high initiative modifiers.

If Spellcraft checks are considered Free Actions, then let's say in a battle between two mages, one who's casting defensively, readying say Magic Missiles to disrupt the casting of offensive spells by the other mage would be a standard tactic.

 

[Alzrius]

It sounds like the scenario you're describing is more about detecting covert spellcasting than actually disrupting it.

To actively disrupt a spell as it's being cast (that is, purposefully disrupting it instead of something happening that does it), you need to basically inflict damage during the actual act of casting the spell. As you noted, the most common way of doing this is with a readied action; simply put, the attacking character is deliberately waiting until the character starts to cast a spell, and then lashes out.

Another way is if the spellcaster provokes an attack of opportunity from threatening characters. Most avoid this by making a Concentration check to cast the spell defensively.

A third method is if the character is still suffering continuous damage from some effect when they start to cast the spell. If you directly hit a character with alchemist's fire, and later in that same round they want to cast a spell, they're still taking continuous damage because they have to take another 1d6 points of fire damage the next round also - that'd possibly disrupt their spellcasting.

Presumably, if a character had an immediate action that could disrupt the spellcaster, they could elect to use it at the moment he began casting. That's debatable though.

Beyond that, it's pretty difficult to actively stop a spellcaster from casting a spell while he's in the process of casting it.

Regarding the scenario you described, bear in mind that normal spellcasting is not a covert affair. A character's somatic components are obvious motions, and his voice is spoken at normal volume (there are some feats and skill tricks (Complete Scoundrel) that can alter this); hence, the passing merchant and his bodyguards would have gotten a Spot check as you say vs. the elf's Hide check, but since "people talking" is a DC 0 Listen check, with a +5 modifier to the DC since your elf is 50 feet away, they will hear him casting the spell - since the notation says that if you beat that DC by 10 or more (e.g. DC 15 in this case) then you can understand what's being said. I'd rule that if they thus make a 15 on their Listen checks, they can realize it's magical speech, and thus make a Spellcraft check (DC 17 for detect thoughts) to realize what spell is being cast.

Also, note that if they make their saves against having their thoughts detected, they know someone tried to use magic against them (though not who, or for what).

 

[taliesin15]

alzrius writes

Regarding the scenario you described, bear in mind that normal spellcasting is not a covert affair. A character's somatic components are obvious motions, and his voice is spoken at normal volume (there are some feats and skill tricks (Complete Scoundrel) that can alter this); hence, the passing merchant and his bodyguards would have gotten a Spot check as you say vs. the elf's Hide check, but since "people talking" is a DC 0 Listen check, with a +5 modifier to the DC since your elf is 50 feet away, they will hear him casting the spell - since the notation says that if you beat that DC by 10 or more (e.g. DC 15 in this case) then you can understand what's being said. I'd rule that if they thus make a 15 on their Listen checks, they can realize it's magical speech, and thus make a Spellcraft check (DC 17 for detect thoughts) to realize what spell is being cast.

if the spell was being cast in a quiet place and the elf was not trying to hide, I would also make the DCs that low as well; however, it is an urban setting, where there is plenty of distracting noise and other things to spot--not impossible to hear or spot the elf, even with bodyguards on the lookout, so I would rate the DC higher than that

Also, note that if they make their saves against having their thoughts detected, they know someone tried to use magic against them (though not who, or for what).

but if they fail their save, they have no way of knowing a spell has been cast?

 

[Vegepygmy]

but if they fail their save, they have no way of knowing a spell has been cast?

Pretty much, yes.

 

[irdeggman]

Rules Compendium pg 117

"Unobtrusive Spellcasting"

Basically use a sleight of hand check against any observer's spot check to mutter words under your breath and make hand gestures surreptitiously.

You can always add in circumstance bonuses for environmental concerns.

But note that unless being observed someone can't tell what spell you are casting.