D&D 5E Blindsight in 5e

[Nifft]

BLINDSIGHT

A monster with blindsight can perceive its surroundings without relying on sight, within a specific radius.

Creatures without eyes, such as grimlocks and gray oozes, typically have this special sense, as do creatures with echolocation or heightened senses, such as bats and true dragons.

If a monster is naturally blind, it has a parenthetical note to this effect, indicating that the radius of its blindsight defines the maximum range of its perception.

This is a very general ability. It's used to represent a variety of monster senses, including echolocation and unnaturally accurate scent. Unfortunately, it's a bit over-general, and it's not clear how it actually works under a variety of conditions.

Similar abilities, for reference:

DARKVISION
A monster with darkvision can see in the dark within a specific radius. (...)

TREMORSENSE
A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius (...)

TRUESIGHT
A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures (...)

Darkvision is pretty clear. It allows you to see in the dark out to a certain range. You're limited to whatever information sight could provide. Darkvision works through a window or wall of force or even a mirror, allowing a spy to peek around a corner with a mirror just as she could under lighted conditions.

Truesight is similarly clear. It's a type of sight, and it penetrates a bunch of deceptive or inhibitory effects which target sight.

Blindsight, in spite of its name, does not provide sight. It allows a creature to perceive its surroundings without relying on sight.

What we MUST assume:
- Blindsight is precise enough to allow a monster to be effective in combat. The text doesn't actually say if Blindsight permits a creature to target enemies precisely, but it must mean that, or it would not be relevant for monsters.
- Blindsight is precise enough to allow a monster to navigate at its full speed.
- Blindsight is precise enough to allow a monster to cook, eat, mate, and maintain its weapons or gear.
- Blindsight seems to penetrate some substances, such as water, just as easily as it penetrates air. There are no special notes on
- Blindsight does not automatically counter Stealth. Doesn't say if it does or doesn't, but monsters with Blindsight do have Perception scores, so it's reasonable to assume they must roll Perception. This is a welcome change from how Blindsight worked in 3.x D&D.


Open questions: (This is basically the stuff that I'm interested in discussing in this thread.)
- Does Blindsight work like vision through transparent or invisible barriers? (Wall of Force, a glass window, etc.)
- How about thin, porous barriers? (A paper wall, a cloth tent, etc.)
- Can Blindsight see around corners? Not using a mirror, presumably, but instead using acoustic reverberations or scent or whatever.
- How about over walls or hedges? An open-top stone or hedge maze could be very challenging against the right opponent(s).


Other notes:
- Bats are explicitly using only echolocation.
- Darkmantles are explicitly using only echolocation.
- Grimlocks can use either hearing or scent as Blindsight vectors. Both are equally effective without the other.
- Other creatures, including Dragons and Half-Dragons, can use Blindsight with no particular vector.
- Firebeetles have Blindsight, which is weird, because their biology expends energy to create light.
- Crabs have Blindsight, which I did not expect. Now I kinda want a crab familiar.

 

[pukunui]

I think it would've been better if they'd called it "blindsense".

 

[Nifft]

I think it would've made more sense if they'd called it "blindsense".

I think you might be making a pun.

Just in case you're not, the reason why they didn't is probably that in 3.x D&D, blindsense meant that the monster knew what tile you were in, but you still had full concealment. In contrast, blindsight meant you did not get concealment.

So the 5e version is closer to 3.x blindsight.

 

[Nifft]

TREMORSENSE

A monster with tremorsense can 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. Many burrowing creatures, such as ankhegs and umber hulks, have this special sense.

Note 1:

Tremorsense relies on vibrations.

Sound is a vibration.

Stealth means avoiding making sounds, which means avoiding making at least one kind of vibration.

The text for Tremorsense does not state that it negates Stealth, and creatures with Tremorsense do have an ordinary Perception score.

This makes me think that Stealth works against Tremorsense.


Note 2:

Tremorsense requires that "the monster and the source of the vibrations are in contact with the same ground or substance".

This might mean that climbing a boulder or a tree makes you invisible to underground ankhegs. That'd be kinda cool, but is it the intended effect?

What if you're attacked by an ankheg while golfing, can you hide in a sand trap? That's a different "substance" from the nearby ground.

Being indoors -- specifically, being in a rigid wood-and-brick-and-tiles structure like a house -- makes a strict reading kinda problematic. In some houses I've visited, I can pinpoint people walking around downstairs even without any particular magical senses. Being downstairs, though, means NOT "on the same ground" -- they're on a separate dungeon level.

I'd suggest something about rigid structures and solidly connected components counting as a single "ground or substance".

Stepping up onto a wooden platform which is anchored in the ground: IMHO this should count as being visible, even though it's not the same substance or ground-level.

IMHO, stepping up onto a carriage or an elephant's howdah would make you invisible, but the carriage itself or elephant would be visible.

Thoughts?

 

[FormerlyHemlock]

Stealth means avoiding making sounds, which means avoiding making at least one kind of vibration.

The text for Tremorsense does not state that it negates Stealth, and creatures with Tremorsense do have an ordinary Perception score. *snip* Thoughts?

Say that Tremorsense is intended to negate Stealth within the sensory radius (because you can see around corners, etc.). Why would that change the Perception score? You'd still need Perception as usual to detect creatures outside your Tremorsense radius, as well as creatures who are not touching the same surface you are touching.

Therefore, I don't think the Perception score being ordinary is evidence one way or the other. It's fully consistent with both hypotheses.

I think this is all mostly a function of how you think Stealth works. If you think Stealth means choosing the right moment to dart between rocks, while the enemy is momentarily looking in another direction, Tremorsense is going to shut that one down cold because it can detect you even behind the rock. But if you think Stealth is about moving so delicately over popcorn that not a single kernel crunches and not a footprint is left behind, Tremorsense won't detect that at all. DM's call (or table's call, if you set rules cooperatively as a group).

 

[Nifft]

Therefore, I don't think the Perception score being ordinary is evidence one way or the other. It's fully consistent with both hypotheses.

Yeah, I can't find any MM creatures which have Tremorsense and no other senses. So that's consistent, at least until we get more monster examples.

If you think Stealth means choosing the right moment to dart between rocks, while the enemy is momentarily looking in another direction, Tremorsense is going to shut that one down cold because it can detect you even behind the rock. But if you think Stealth is about moving so delicately over popcorn that not a single kernel crunches and not a footprint is left behind, Tremorsense won't detect that at all.

I don't think it's useful to take something as abstract as Stealth and force it to be only one of those two specific interpretations.

I think it's obviously both of those, plus more -- stuff like knowing how to pick boots which don't squeak, being able to intuitively guess where you can or can't be seen from several perspectives simultaneously, ability to glance at rubble and know where you can step without setting off a landslide or otherwise making noise, and so on.

 

[FormerlyHemlock]

I don't think it's useful to take something as abstract as Stealth and force it to be only one of those two specific interpretations.

I think it's obviously both of those, plus more -- stuff like knowing how to pick boots which don't squeak, being able to intuitively guess where you can or can't be seen from several perspectives simultaneously, ability to glance at rubble and know where you can step without setting off a landslide or otherwise making noise, and so on.

Sure, of course. No argument there, Stealth can cover lots of activities, and I have PCs make Intelligence (Stealth) checks to evade enemy patrols cross-country by avoiding obvious chokepoints.

But I don't think that "boots which don't squeak" and "walking that wouldn't set off a pressure sensor" have to be the same thing. I think that is up to DM judgment/table judgment.

 

[Nifft]

But I don't think that "boots which don't squeak" and "walking that wouldn't set off a pressure sensor" have to be the same thing.

Pressure sensor?

I think you're asking if Tremorsense is the same as a trap trigger, is that correct?

If so, I don't think that relates to anything I've said. Vibrations and mass (or pressure) are not the same thing, and Tremorsense does not detect mass. It detects "vibrations", which are presumably caused by movement across a surface.

Like I said previously, it's already in the rules that Stealth can negate one type of very common vibration which is caused by movement across a surface -- that's sound vibrations, which Stealth can either allow you to avoid, or mask with another noise via timing, or disguise as something else, or whatever. I'm not particularly married to any one interpretation. But it's very clear that Stealth can negate Perception of sound vibrations, and IMHO that leaves the door open for Stealth to negate Perception by this other kind of vibration.

- - -

At my table, I'd require that the player declare that she's trying to use Stealth to fool Tremorsense specifically, rather than conventional senses. Expanding on that, I'd allow a Stealth roll vs. Blindsight, and the declaration would need to be specific to one known creature's Blindsight.

 

[FormerlyHemlock]

Pressure sensor?

I think you're asking if Tremorsense is the same as a trap trigger, is that correct?

Observing, not asking.

I agree that "vibrations" is unclear, and I think that requiring the player to specifically focus on fooling Tremorsense as opposed to sight is a good idea. But it would still be up to DM/table judgment, because some of the things that work against sound ("or mask with another noise via timing, or disguise as something else") won't work against Tremorsense. It's up to the DM which of those techniques Stealth relies on.

 

[Nifft]

Observing, not asking.

Hmm. Well, Stealth and "disabling a trap" are both Dexterity-related, but "disabling a trap" does not benefit from proficiency in Stealth, so I honestly don't see how Stealth would necessarily help avoid that. If you think my post implied such, let me know where I said that, since that's not a thing I'd intended to imply.

because some of the things that work against sound ("or mask with another noise via timing, or disguise as something else") won't work against Tremorsense.

Nah, it's pretty easy to come up with a creative way for that stuff to work.

For just one example, consider the previous case where Stealth was used to move up an unstable heap of rubble without causing a minor landslide. Working against Tremorsense would do the exact opposite: cause rocks to fall -- and then bounce, with the approximate cadence of a human stride. The sneaky character then Stealths along, matching the vibrational profile of a bouncing rock, and therefore entirely undetected as a person rather than a rock.

That's just one example. There are many other possibilities. A good group of creative players can fluff any legal skill interaction.