Invisibility

Invisibility is pretty neat, and very handy for adventurers and such to be using. Obviously, things would have gone a lot differently for Bilbo without such a handy little magical ability attached (for some bizarre reason) to Sauron's jewelry. Seriously. It was Sauron's ring, not one of the ones he gave to others. This was his personal icon of power, and it grants invisibility and not raging beams of fiery doom and army battering waves of force? You can't just wiggle your finger and summon the nazgul? I mean, I'm sure there are depths to it that Frodo and Bilbo could never have accessed as untrained bumbling hobbits, but still. 

Anyway, so invisibility. Handy, but not a guarantee of safety or stealth. Definitely very cool when we're talking about greater invisibility though, as now you can attack and remain invisible. However, with either version of the spell, you can still be heard, and canny adventurers can easily do things to figure out your location, even if you've had the foresight to center a silence spell on yourself or the area being infiltrated. There's glitterdust of course, along with fairly low tech solutions like scattering something on the ground. 

One thing that occurred to me recently was detect magic. I don't know if I'm slow or what, but the other day it hit me like a ton of bricks. Detect magic. Invisibility is magic, detect magic detects magic. Boom! Mind blown. 

So most Wizards have cantrips, and detect magic is one of them. Cantrips can be used as many times a day as a Wizard likes, as long as it was one memorized at some point after a rest. I think there are rules that allow you to memorize different spells over the course of the day too. Not sure on that though. Anyway, so yeah, detect magic doesn't rely on visual senses and that sort of thing, it just detects magical emanations. So it's kind of handy against stuff like illusions or invisibility where you can't really trust the eyes of your character. 

It's a little slower that just throwing down piles of salt or buckets of mud or other stuff to show you where your invisible foe is, but it's better than nothing if you don't have any other options at the moment. By slower I mean that you have to concentrate for three rounds before you can sense the location of magic auras in the area, and you also have to concentrate on maintaining the detection (which I did not know during the adventure in the Tomb of Horrors, so whoopsie there), which is a standard action. So it takes some warm up time and it prevents your caster from casting offensive spells or stuff like purge invisibility, but in a situation where you don't have a better method on hand, it might be handier than just flailing around wildly at a moving and invisible target. Regardless, it's not perfect, as only the person using detect magic can sense the location, but that at least allows someone to call out where precisely to attack. 

Now the rules for invisibility state that anyone can sense an invisible creature within 30 ft. with a DC 20 Perception check, but that just tells you someone is somewhere within 30 feet of you. It's a DC 40 Perception check to pinpoint their location, and even if you do, they still have total concealment, which confers a 50% miss chance on all attacks against them. With someone using detect magic and calling out locations while others are attacking I would probably resolve it something like the aid another action, a +2 to attack rolls and the skill checks to pinpoint the location. Detect magic only locates the aura, it doesn't tell the user how the invisible creature is moving or anything like that, and saying "He's over there!" is different than being able to see precisely where he is and what he's doing with his feet. I think I'd reduce the total concealment to concealment, which still confers a 20% miss chance on attacks. 

Honestly, detect magic isn't super useful and awesomesauce in this context. But, it's better than flailing wildly and attempting to get a DC 40 Perception check while wishing you'd brought that bag of marbles with you to work today. Eran, one of the premiere percepters of the Orcunraytrel group, would need to roll a 25 to pinpoint the location of an invisible creature. 

Perhaps of more use would be the ability to defeat illusions using detect magic. I don't bother with illusions much, it seems like there's too much opportunity to get everyone butthurt about what constitutes interacting with it and disbelief. The one I have used in the campaign was a black stone wall in a place full of black stone walls. Seems reasonable that it would be there, so no real issue with the issue of disbelief and saving throws, and it wasn't exactly hostile. But you know, tombs and ruins full of fell magic could have illusions in them. It would be prudent to wander into a room and concentrate for three rounds to see if you find any illusion auras before poking around in corners. 

I dunno, I don't feel like this way of using detect magic is particularly innovative. I just never considered it until recently. But, I don't have too much practical experience with spells and the tactics of using them extensively.