This article was first broadcast in Episode One Hundred Ninety Three on 22nd December 2021.
Killer DM: All right, up you get.
Ostron: Uggh. First, why are you here, second, why did you knock me out?
Killer DM: Because I needed you alive. Note the past tense there; you’re going to have to keep proving your usefulness from here on.
Lennon: All right, it’s been three hours and we need Ostron awake…which he is? I thought you were talking to ROSTRO?
Killer DM: …We had a fight.
Lennon (wonderingly): Oh my goodness what’s that even like?
Ostron: Oh did you break the machine? You know how long I worked on that?
Killer DM: It’s fine. It is possible to have disagreements without resorting to violence, you know.
Killer DM (after a pause): What?
Lennon: I’m just…adding another entry into the list of ‘things I was not expecting this morning’.
Ostron: What did you even need ROSTRO for, anyway?
Killer DM: Well I want to have a purple worm eat my players because that’s both classic and fun but its entry says it has tremorsense and blindsight but with different ranges. I was trying to figure out the difference and all ROSTRO could do is spit back the rules entries at me.
Ostron: Well that’s…sort of what I designed it for.
Killer DM: Then we got into “how does this compare with darkvision” and it was still just spitting back rules and it was a whole thing.
Ostron: Yeah, so working with the differences and the nuances there requires some…creativity that I didn’t really build into ROSTRO’s makeup.
Killer DM: I appreciate that. People getting too creative usually inspires me to get creative with piles of rocks.
Ostron: Yes, and we like to avoid that. But maybe we can help in this case. Let’s review what we’re starting with first of all.
Creatures in D&D are generally considered to have all of the same senses real humans do by default; they can see, hear, taste, smell, and touch with equal ability. In reality there are countless nuances and variations on that scale. Some creatures can see longer distances and process information faster, a lot of animals have far more sensitive hearing than humans, and almost everything is able to process scents and smells better than most humans. But for the actual rules of D&D, alternate senses are only called out when they move far beyond what the regular senses are able to do.
Four discrete senses are possible: Blindsight, Darkvision, Tremorsense, and Truesight.
We’ll actually start with Darkvision, as that’s the one that comes up the most. The rules as written state that “A monster with darkvision can see in the dark within a specific radius. The monster can see in dim light within the radius as if it were bright light, and in darkness as if it were dim light. The monster can’t discern color in darkness, only shades of gray.”
I’m glad we started here because I have a few things that I need to bring up. Darkvision does not mean people can see in the dark just fine. The rules up there say you can see in darkness as if it were dim light. I had to listen to ROSTRO tell me multiple times that dim light means everything is lightly obscured. Lightly obscured is one of those phrases with actual rules behind it, in this case it means any time you want to look at something you’ve got disadvantage.
If you look back at earlier editions, Darkvision was either night vision or infrared vision. You’re probably going to be able to see corridors and people if they’re in front of you and in the open, but details of anything are going to be hard to make out and take extra time to examine. And forget reading, by the way; people who use modern night vision all say reading is no fun that way and requires intense concentration and the ability to tolerate intense eye strain.
If you’re the DM, it’s up to you how much you want to play up the negative effects that persist with darkvision. In reality it’s not going to inhibit things too much unless a lot of critical activities require good sight or perception checks. It does mean filling a dark corridor with hidden creatures and traps is still going to be a nightmare even if the entire party decided the dragonborn, human, and the halfling would guard the door. Particularly if the DM decides that sight is a key component in disabling any traps the party happens to be lucky enough to locate.
Lennon: And yes, it goes without saying we know that’s exactly what you would do.
Killer DM: I keep telling you we’re on the same wavelength. And I also keep telling you to find another one, it’s creepy.
Lennon: Now we get into the odder forms of exotic senses. We’ll start with Tremorsense because it’s slightly simpler. Again pulling from the rules, tremorsense is used to “detect and pinpoint the origin of vibrations within a specific radius, provided that the monster and the source of the vibrations are in contact with the same ground or substance. Tremorsense can’t be used to detect flying or incorporeal creatures.”
Killer DM: Before Ostron puts us to sleep I want to get a rules lawyery argument out of the way; tremorsense can’t detect flying creatures while they are literally flying in the air. Just being able to fly doesn’t make you invisible to tremorsense. As soon as your aarakocra touches down on a solid surface you’re pillow stuffing.
Ostron: Thank you for that mental image.
However, she’s right; tremorsense’s major mechanic is vibration. The part where a lot of issues arise, however, is it’s not really specified what actions cause vibrations, at least not in the mechanics of fifth edition. However, if you go back to the crunchier edition 3.5 there’s some more clarification of intent. In the 3.5 SRD, it stated “[the creature] must itself be in contact with the ground, and the [other] creatures must be moving. As long as the other creatures are taking physical actions, including casting spells with somatic components, they’re considered moving; they don’t have to move from place to place for a creature with tremorsense to detect them.”
From a mechanical perspective, that’s an important clarification because it limits the vibrations that can be sensed to ones based on movement. Anyone who’s taken physics or watched enough submarine movies knows that sound is actually measurable as vibrations, but for the purposes of the D&D rules that doesn’t seem to count. So you can theoretically hold an entire debate right above a burrowed Kruthik nest and they won’t notice, but as soon as someone slams their hand down on the table in frustration it’s game on.
Mechanically speaking all that has to happen to avoid making a vibration is the creature does nothing with their action or move action but if you want to inject some realism into things you can have them make a check. If you don’t believe that’s fair, try sitting or standing somewhere and remaining absolutely still for any length of time. Some people might be pretty good at it, but if you’re a fidgeter you’re worm food.
Now we come to my least favorite of these; blindsight. The rules here are just oh so helpful. “A monster with blindsight can perceive its surroundings without relying on sight, within a specific radius.” That’s it, that’s the whole explanation. But never fear, Jeremy Crawford himself stepped in to help. Here’s his contribution: “Blindsight lets you perceive your surroundings, including environmental phenomena, yet a phenomenon that impedes only sight (it doesn’t provide cover) doesn’t work against blindsight.”
Killer DM: If I ever meet that man, we’re going to have words. Most of his words will probably be things like “Stop it, please, I’ll do anything,” and “oh the pain, make the pain stop.”
Lennon: So before relying on torture, we can figure out some of the intent behind the rules by looking at 3.5 again.
Killer DM: Who said the torture was to get rules clarification?
Lennon: I really miss Ryu when she’s not around.
Killer DM: Oh, she’s not on the side of this argument you think she is.
All right, moving on from that as quickly as possible, the description of blindsight in 3.5 says “Using nonvisual senses, such as sensitivity to vibrations, keen smell, acute hearing, or echolocation, a creature with blindsight maneuvers and fights as well as a sighted creature. Invisibility, darkness, and most kinds of concealment are irrelevant, though the creature must have line of effect to a creature or object to discern that creature or object.”
It further clarifies:
Blindsight never allows a creature to distinguish color or visual contrast. A creature cannot read with blindsight.
Blinding attacks do not penalize creatures using blindsight.
Deafening attacks thwart blindsight if it relies on hearing.
Blindsight works underwater but not in a vacuum.
Again, while that clears things up more than the basic entry for fifth edition, it also creates some more questions because of what’s left out of 5th edition rules. None of the monster stat blocks specify how a creature’s blindsight actually functions. If you go into the descriptions for some of the monsters you can get more info. For example, in the entry for the Grell it says “They have keen hearing…and their skin is sensitive to vibrations and electrical fields, allowing them to detect the presence of creatures and objects in their immediate vicinity.” However, Gelatinous cubes also have blindsight and there’s no indication in the monster manual or any other fifth edition resource whether they can smell, sense vibrations, or are attracted to the sound of hair growing on someone’s skin. You have to go all the way back to 2nd edition to find a reference to gelatinous cubes sensing vibrations and being attracted to heat.
Now again, the intent with 5th edition is to simplify things. Mechanically, blindsight only means that at whatever distance the creature’s blindsight works, it’s impossible to impose the blinded condition on them, and they’re still able to see regardless of light level or any level of an object being obscured by the rules. Throwing a blanket over a gelatinous cube does nothing to hide you from it. Also you can probably say goodbye to the blanket.
But of course you have the people, some of them on both sides of the screen, who like their fictional worlds to function beyond the basic rules. I’m one of those people, as long as the ways we go beyond the rules benefit me. For example, the fact that the gelatinous cube hunts by sensing heat is great; to me it says that you can’t really make a stealth check to hide from it unless you’re also willing to let your buddy hit you with ray of frost just beforehand.
But of course that is a difficult road to travel. As anyone that’s played D&D with an engineer can tell you, as soon as you allow real world physics and logic into your games there will be a lot of things you have to suddenly account for. For example, the other thing most editions say about the gelatinous cube is that your average specimen weighs 50,000 pounds. So all the characters have to do at that point is find a nice rope bridge to run across and the cube can’t follow, even if all the rules as written say it’s perfectly capable of moving across it. That’s why all rope bridges in my games have a weight tolerance of about 2 pounds. If I can’t use it, nobody can.
There are numerous other ways that those arguments can devolve as well. For example, if you’ve got a creature like a bat or a dolphin that relies on echolocation and you manage to do something that deafens it, does that also remove the blindsight because it works based on hearing?
It’s entirely up to you whether you want to delve that deeply into figuring out how different creatures’ blindsight functions and what can sabotage it. If you’re the DM just make sure you’re consistent with it. If you don’t let the characters deafen your vampire bats to blind them in a cave, you can’t then do it to one of their bat familiars later to prevent them scouting ahead.
You totally can. You just have to implement an “arguments with the DM are punishable by death” policy at the table.
Truesight is at least easier to deal with because it’s a made up ability created to deal with made up abilities. Basically it just negates anything magical that messes with sight. From the rules, “A monster with truesight can…see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the monster can see into the Ethereal Plane”
Fortunately this is the one area where all the uses and conditions are pretty well spelled out. Note that truesight does not make them immune to blindness; it just enhances their sight well beyond normal.
At this point, there are no player character races that have tremorsense or blindsight by default, though both of them can be gained if the characters end up grabbing some specific magic items. The way Ostron would say to deal with the problem is to take the rules as written very strictly and, where there is confusion like what qualifies as a “vibration” for tremorsense, go back and look at the 3.5 rules. My solution is just don’t use monsters with those abilities and make sure they never find any of those magic items.
Wow. I mean, that’s kind of black-and-white but it’s very reasonable for you.
Good. Because my actual solution is to do what I want and kill anyone who argues, but I’ve been told that’s…mean.
Anyway, as the DM you set the tone for how much you want to deal with any of this. If you want tremorsense and blindsight to be more nuanced with a few different ways to overcome them and more specifics about how they work, that’s your call. As we said, though, make sure you’re consistent with how you implement all that or you’ll have whiny players at your table.
If you’re a player, respect what level the DM is working with. If they say that the beholder can scream at you from across the cavern and not attract the purple worm you’ve been running from for 20 rounds, that’s the way it is. The fact that you know 80 decibels of sound is enough to start a lot of things vibrating in the cave isn’t relevant unless the DM says it is.
Lennon: Where did you get 80 decibels?
Ostron: That’s about how loud someone shouting is.
Lennon: I know that, why does she?
Killer DM: Fighting with ROSTRO. For hours. Keep up, limey. Anyway, again, let the DM determine how crunchy the rules are getting and respect that. Or they might decide how crunchy you get when the fireball hits.
Lennon: Are we all happy with the rules now?
Killer DM: Happy is overselling it but I suppose I can live with what I know.
Killer DM (with false enthusiasm): Hey Ostron, do you want to hear about the discussion where ROSTRO gave me all those decibel measurements?
Ostron: Oh, yeah I (yawning) …Don’t know how I didn’t…see…
Lennon: Hey! Hey! No! You had your fun.
Killer DM: I had a fight. That wasn’t fun.
Lennon: That sounds like a “you” problem. I came in here in the first place because I need Ostron for the scrying pool. Now we’re late. It’d be nice if you could doff your cap as well, by the way; the listeners probably want dragon squees more than “12 tips for barbecuing the barbarian.”
Killer DM: Hey! Who told you about that title!? That’s on the short list for my self help book!
(Lennon groaning in misery)