After playing in a game recently (as well as when designing my own campaigns) I ended up having a few questions about exactly how the greater detect magic spell works.
1) When looking at a lingering aura from a spell, is it possible to identify exactly what spell was used? And if so, what is would the DC be?
I know that Spellcraft can be used to identify a spell as its cast or identify a magic item, but a lingering aura doesn’t seem to satisfy either of those. The closest I can find to help would be a Knowledge(arcana) to determine the spell’s school, but that still only narrows it down to a large category of spells.
2) How does greater detect magic interact with attempting to identify the spellcasting signature of spells cast using items? Does it register as the item creator’s signature, item user’s signature, or simply fail to have a unique signature? Does this change based on if the item used was a spell-completion, spell-trigger, command word, or use activated item? And does it affect the lingering traces left in a creature’s aura to determine their last spell casted?
During the last session I played in, my GM ended up making a quick ruling that I simply failed to find any unique signature on the spell I was attempting to look into (because it was done with a spell-trigger item) – comparing it to “trying to analyze the handwriting of a printed letter.”
On the other hand, as a GM planning to use this spell for law enforcement in my own games later – it’d seem pretty crappy if buying/making a scroll/wand was all it took to defeat the spell. So I’d really like to know what the official interpretation is supposed to be.
3) When looking into the greater magic aura spell, am I correct in seeing that it is only capable of targeting objects/creatures, and not spells themselves, thus making it impossible to mask the traces of any spells used over a great distance?
4) Finally, I’m assuming greater detect magic pierces mask dweomer as mask dweomer states that more spells powerful than detect magic pierce it with a successful Will save (though greater detect magic only specifically mentions magic aura). However, is there any greater version of mask dweomer in the same vein as greater magic aura?
Right now I’m playing a sneaky Vigilante Cabalist (uses witch spell list), so seeing that I can’t get access to greater magic aura is annoying for me if there isn’t a similar version for mask dweomer or another way I can invest in to try to hide from greater detect magic.
1) You can learn the same information from a lingering aura as you can with a spell that is currently there. You can make a knowledge arcana to identify what arcane school it came from, and if it is from a magic item you can make a spellcraft to identify its properties.
"A magical aura lingers after its original source dissipates". The magical aura is the same, it's simply dim now instead of whatever the original aura power was.
2) I believe you would identify the creator of the magic item (unless it was a staff).
3) No, you can still detect spells with them. Greater detect magic does everything that detect magic does, as well as doing more and doing it better.
4) Yes, that assumption is correct. I am now aware of a greater version of mask dweomer.
You want to hide from detection spells? AMulet of Proof against Detection and Location. It's pricy but does what you want, and it works against all divination spells. They do get a check though.