Senses in D&D and other fantasy rpg's have, in my humble opinion, left a lot to be desired. Old editions of D&D where too scientific, using infra and ultravision for example. Newer editions simplified things and just use terms like Darkvision, that simply means you can see in the dark. This seems to me to be mostly to give you an mechanical benefit, which again doesn't add to an immersive and interesting game.
What you can detect, how and under what circumstances in the real world are determined by physics which means that even the fanciest way of detecting things comes with limitations and drawbacks. This makes real world combat tricky, uncertain and multifaceted, I want this in my fantasy games as well. So how to go about achieving that?
If real sensors are based in real world physics, senses in a fantasy world should be based on the physics of that world. Sounds simple in principle, but can be overly complicated to implement, and we want something that can be understood and usable during game play. I'm going to try and have a go at such a way of looking at senses in my Greyhawk games. The goal is to have a handful of different senses that makes sense (pun very much intended..lol), are familiar to D&D players, enhance tactical and flavor to encounters, and doesn't bog down play too much. These are lofty goals, lets have a go to try and work towards those goals.
Senses need to work with what the world are made out of, so time for a deep dive into the inner workings of the multiverse in my campaign.
There are many different planes, but the stuff (or other non substantive matter) they are made of can be group into a few different types:
These planes are made out of real stuff, which comes in different forms and dimensions (more on that later). In this category we find the Prime Material Plane, the Inner Planes, the Ethereal Plane and the Positive and Negative energy Planes.
These planes are are only "semi real" being made up by the thoughts, feeling and pure power of living creatures. These planes have then started to take on a life of themselves, which makes them semi real. Here we find the Feywild, Shadowdark, Astral and Outer Planes.
Mental planes doesn't exist for "real" only as a places you can go to mentally. They can still be deadly and possess a number of traits.
Senses in a fantasy world need to work within the parameters of this multiverse. This is where it starts to get interesting, what kind of senses would the Metaphysical make possible? The Ethereal?
In order to make it simple, normal vison, hearing and smell need to work as we know it in our real world, with more sensitive version available, like low light vision and keen smell. These senses works within the Physical, and they have a hard time detecting magic like illusions, life, alignment and the true nature of things, this is the realm of other forms of senses.
Works just like normal vision but is more sensitive and needs less light and effectively doubles the distance you can see using a light source, making mon and even starlight function like daylight.
Also works in the Physical, but in the Ethereal Dimension and sense the presence of "substance". It is a very primitive form of Ethereal sensing and make you detect your surroundings within a narrow range (usually 30-120 feet). No colors and very limited texture are perceived. Rain, fog or clouds limit Darkvision in a similar way to normal vision. Darkvision works even in bright light since it is based on detecting the presence of substance. The lack of detail makes it less reliable and creatures relying on it has Disadvantage on Perception checks.
Darkvision works on all Physical Planes and are usually limited to around half or less on Metaphysical Planes. It doesn't work in Mental planes and some demiplanes that lacks real substance.
Most creatures that have both Normal vision and Darkvision have to take an action to shift from one to the other, certain feats my reduce this to a bonus action. some monsters have separate senses and can use both normal and darkvision at the same time making them formidable hunters at dawn and dusk.
A more advanced version of Darkvision that have no range limit. Deep Darkvision has the same limitations as normal darkvision (except range), on the Prime Material Plane. On Metaphysical planes it is reduced to a max range determined by the nature of the plane. It doesn't work on the Inner Planes and Mental Planes.
Ethereal Vision (sometimes ranged)
This is more powerful than the Darkvisons, but comes with its own limitations. Ethereal vision lets you see the substance of the world, where things are, and also traces of where stuff have been. You can "see" the traces of things that moved from a place. This can be seen as very powerful, the drawback is that it requires "sampling time" so it can't be used in real time. In game terms this means a creature that uses Ethereal vision to sense its surroundings need to spend an action and can then comprehend the world, which makes it near impossible to use in combat but great for mystery solving and tracking. Ethereal vison also makes ghosts, phasing monsters and others using the Ethereal becoming visible.
Ethereal Vision works on the Ethereal Plane of course, the Prime, and some Metaphysical Planes. Doesn't work on the Inner Planes and on Mental Planes.
Nothing stops a nefarious DM from having monsters that are better at relaying on Ethereal Vision and can use it in real time. Creatures native to the Ethereal Plane, flying creatures like dragons and others who need to "see" well regardless of lighting conditions.
Life and Unlife Senses (range)
The Positive and Negative Energy have to have its senses too, and his what every cleric and paladin needs, detecting life and undeath!
You can divide this up, and say that a creature can only detect one of the extremes, like giving undead Life Sense, or a paladin Undeath Sense. Other creatures can detect both its up to you in designing monsters, classes, magic and magic items.
Elemental Sense (range)
This covers the opposite end of the physical from the Ethereal, the Elemental. Here we can have senses for all the different Elements - earth, water, fire and air, and derivative stuff, like the stone cunning of dwarves, and the ability to detect gems, gold and other things. These senses ought to work even through other stuff, but only on the Prime, Inner and with limitations on some of the Outer Planes. You can make it require extra time for detection if you think it is too powerful.
This is the ability to sense your surroundings using vibrations in the ground, air, water or other surrounding medium. This works on the prime, Inner and most Metaphysical Planes. doesn't work in the Ethereal and Mental Planes.
These senses covers the physical side of things and gives us some useful and interesting gaming options. Time to move beyond the limitations of the physical tangible world.
This is an interesting environment that can cover almost anything, the key is to maintain interesting constraints and try to not duplicate any of the physical senses.
Alignment Senses (range)
This is a classical D&D sense, that have caused so much headaches in my earlier days of gaming, probably at your table too. Lets try and make this interesting, make sense while keeping some mystery. Lets start with the Good - Evil.
I see good and evil as traits that creatures that have make use of positive or negative energy to heal or harm life, necromancy, or just harbor ill intentions towards life strong enough to create an "metaphysical aura". Over time these forces combined with entropy (chaos), and the imagination, love, fear and other emotion and attitudes created the astral and outer planes.
This can be be detected as a continuum from Good to Evil, usually felt like a gut feeling but some creatures might be able to see it as a tint. Most creatures are only sensitive to one end of the spectrum, the opposite from themselves. This creates some interesting limitations, for example you can blend in among your own but will stand out among your foes. A demon with detect good can detect a paladin out in the prime (neutral) but he would be undetectable in a good temple. Same goes with aligned creatures and the outer planes who are (usually) aligned Good or Evil, aligned creatures would stand out as beacons of difference trying to invade the opposition and able to hide at home. This is one of the main reasons that battles usually takes place on neutral ground, it is the best for both sides.
Law - Chaos are the other classical D&D axis of alignment, which I treat a bit differently. You can just treat it the same way as Good and Evil if you prefer. I treat is a single polar more or less Chaos. Chaos is the intention of trying to physically, or mentally, destroy the world around you. This disturbance can be sensed, and even some planes are chaotic by nature.
When you leave the neutral Prime and head for the alignment infused outer planes alignment sense might become the standard way to perceive the world. It is the way for identifications, weed out intruders and even finding your way around. The way to blend in is to adjust your attitudes, intentions and behavior or risk being hunted by things like Bebiliths in the Abyss where its alignment sense works at vast ranges across a layer even if it is open. The same Bebilith on the Prime might still be able to detect you, but only at a close range and might even be effectively blind apart from seeing alignment auras.
By using planar physics, and letting it guide what senses a creatures has we can create some outlandish creepy stuff that, in my opinion, feels fantasy and can provide for interesting games.
Arcane Sense (special)
This is the ability to sense magic (I leave the details on how to those who know more than me!). Its something that can be done through magic, so why not give some creatures the ability to do so naturally. The key here is to give it limitations that are interesting. Make the range dependent on the strength of the magic, but it should be substantial as a deterrence for powerful creatures using magic to solve everything. I have it a mile per level of magic, cantrips limited to 100 yards or so. Also have it blocked by matter, so it has to be within line of sight (or effect), which is a good reason for wizards (and other powerful creatures) to build towers or nest on mountain tops.
How much information can you detect? I have it that it is an imprecise sense, and you can detect perceived strength. Meaning it can be a first level effect a mile away or a ninth level nine miles away. The School and Strain (meaning what type of force used, my house rule) can also be detected but that is about it. It is also a sense that usually require active search. Powerful creatures, like deities, or special magical items might be able to passively detect.
You can also play with having clerics and others who wield divine magic to be detectable when they memorize or otherwise gain spells. Maybe that is true for Arcane casters as well. That is one of the benefits with Primal Magic in my campaign, it is weak, but that makes it hard to detect. you can call for an animal and not be detected by the big evil that always watches...
Detecting Fey and Shadow are variants of the Arcane Sense, specialized on its environments, effects and creatures.
The Metaphysical Senses works in most places, but don't always trust it. Strong Powers of the world can use magic to either bend the results, or just blast out magic energies and drown out lesser magic. Like in so many cases, its only the strongest, or brightest thing that can be detected, the rest are hidden in the fray.
If you want to be specific you might want to divide up magic sense to only detect certain strains or schools of magic.
Spirit and Soul Sense
If you have rules for things like ghosts, souls on its way to their final resting place and similar things they belong in the metaphysical and can be detected. Here you can play with scope, range, precision and how far back in time a spirit that used to haunt this place can be detected, and what can be gleaned from a detection. This is good for spooky sessions, and to have limitations that you as a DM knows but the players have to figure out through trial and error are what makes it fun, and spooky. Combine it with life sense (or better) for the monsters and you can have a fun game.
Special Metaphysical Senses
Have Fey and Shadow based creatures sense their own kind and strangers on their home planes. This can be tied into Spirit and/or Soul Senses if you want fewer senses.
This is most far out there realm, and some campaigns might not use mental planes or senses. It is fertile ground for fantasy story telling and really useful for adventurers, and foes alike.
Detect Thoughts (special)
The very basic of mental sense, useful to detect the presence of other creatures. This need to be limited though, make it line of sight, imprecise and require an action to do. The range of Mental Senses are different they don't work in the real world, it is the mental realm. You might only be able to reach creatures you know and can think about, regardless of distance. You might be able to detect anyone's thoughts, but it is very hard to sift them out from all the mental "chatter", unless they are by themselves. If this is the case you hide best against this form of sense in crowds, you are vulnerable by yourself.
Advanced Mental Senses
More advanced versions of this can be interesting, like sensing intelligence as well as thoughts. Sense created minds like animated creatures for example, or detect special emotions in a crowd are other possibilities. Again make sure to include limitations in range, blocking, countermeasures, effort needed and so on.
Mental Senses, and even advanced versions might be needed to not be effectively blind on Mental planes. The range might be based on the creatures mental abilities. Intelligence and/or Wisdom might determine the range, charisma might make you stand out for example. Emotions and intentions are key in this realm, trying to hurt someone might be detectable, as well as fear and affection.
Now we have covered the different base forms of existence, what more can be left?
What if the Elven reverie they, instead of sleep is in some form of communication, communal defense, getting to know the world or training their Fey talents.
Time to look into the truly powerful and their special ways of sensing the world. When your character goes up against a ancient dragon wyrm, or worse, a deity, what special tricks do they have up their sleeve to stay on top. Just like in modern warfare, information is key to success in fantasy battles.
Truesight is the ability to combine normal-, Darkvision an/or Ethereal with Arcane Sense to see a more comprehensive view of the world. It gives a creature a direct ability to detect illusions, invisibility and incorporeal manifestations within its range. Truesight is hard to fool, using anything except physical barrier.
The drawback is that this limits the creatures vision to the range of its Truesight, making it effectively blind beyond this range unless it has some other form of sense, that is not dependent on its eyes, like tremorsense. A combination of physical and magical cover might be needed. A Fog Cloud or Cloudkill spell might help give you an edge.
Other combinations with Blindsense can be cool, but remember to limit the range and keep track of how it can be fooled. Remember that drowning a sense with information is often easier than to try and hide from it!
Senses, due to their nature also have other quirks that should be taken into account to make encounters more interesting.
Is the information gained from the sense precise enough to be used for targeting, or is it only good enough for a general awareness.
Field of View
D&D doesn't really make use of creatures' facing, except when flanking. In some instances it might be very useful sneak up on an enemy from behind to avoid detection, but what if its Alignment Sense are omnidirectional and i can sense you sneaking up even if you ease your Stealth Check.
Speed and Reaction Time
How long does it take to effectively use, and is it constantly in effective use, or does it require a round (or more) of concentration.
Secret or Open usage
A creature that makes use of a sense, is it done without any detectable behavior or can it be done without its surroundings knowing about it. An example would be reptiles like snakes and some lizards who smell using their tongues. Some creatures might have special posture or movement when using a sense, or make noise blowing air through its nostrils to use its smell.
Some senses might be active, like echo locations or sonar, used by bats and dolphins. Exotic versions of that might be mind blasts to weed out intruders by aboleths or ilithids. Active senses can also combine detection with an attack, like a wyrm's roar being both an echo location sense and a sonic attack stunning its prey.
Exotic versions of this can be available on other planes where real world physics are not as limiting. Creatures native to the Ethereal, Fey, Shadow and other planes might be able to use the very environment to detect, confuse, hinder and even harm. Mental and Demi planes might be particularly dangerous when faced with an antagonist who literary have created the plane in question.
This was first take at trying to make Sense in the world of Fantasy. I hope it has given you some insights and ideas that can help you in your games!