The 5th edition of Dungeons & Dragons was written with the specified intention to use simple language and not to get too caught up in excessively¬† granular definitions. Which is generally a good idea, given how fiddly the 3rd edition was, and how dissociated the mechanics of the 4th edition were perceived. GMs are supposed to use common sense, or make a ruling about how some unusual interactions between two rules should work out in their campaign. That’s how most games other than 3rd an 4th edition work. But of course, there will be some casualties along the way. Blindight is one of them.

The definition of blindsight is terrible. “A creature with blindsight can perceive its surroundings without relying on sight, within a specific radius.”

Refuses to elaborate further.

Goes to printers.

That really doesn’t tell us anything! “Blindsight is a sense”, it is telling us, but nothing more. It really is completely useless. What blindsight actually is and what it does is something GMs have to make up themselves. I always feel that a good starting point is to go back to older versions of the rules and look at earlier definitions of a mechanic or rule. Of course, writers can always choose to use an old technical term and apply it to a completely new concept, but I think in that case they would have taken more care to explain the new mechanic and not forget to write it down “because it seemed obvious”. 3rd edition fortunately has much more detail on the effects of blindsight, which greatly influence my personal ruling on what it does in my 5th edition games.

The name blindsight implies that it is very similar to sight, but operates even if a creature has no sense of vision. Within the range of the blindsight, we treat the creature as if it could see, even when it can not. The primary effect of this is that within the range of the blindsight, the creature ignores the effects of blindness. Though we have to distinguish here between the mechanical effects of blindness as specified in the game rules, and all the other limitations of not being able to use vision.

When a creature or object is being lightly obscured or heavily obscured, it inhibits the vision of creatures, but apparently nothing else. You can still hear through sources of concealment, shot projectiles through it, or cast spells through it. As such, I would rule that to a creature with blindsight, nothing is ever obscured. (And all that lighting conditions ever do is to make things obscured.) Though I probably would also rule that to a creature whose blindsight specifically comes from its sense of hearing, environmental noise could have the same effect on their blindsight as obscurement does on vision.

Now does blindsight allow a creature to perceive things and other creatures that are behind cover? My ruling is that it does not. If a creature has an exceptional sense of hearing, then it already has advantage on Wisdom (Perception) checks that rely on sound. If a character is trying to hide behind some crates that provide full cover, from a grimlock that isn’t actively searching, I would probably ask the player for a Dexterity (Stealth) check to be extremely quiet. While no check would be needed if it was a goblin instead. Heightened hearing works around corners, but blindsight does not. It’s still called sight, after all.

However, a big question mark still remains and that is the topic of illusions. The spells blur and mirror image both specify that they don’t have any effect¬† on creatures that have blindsight. But no other spells do. The most important one being of course invisibility. In 3rd edition, things are very clear. The description of blindsight says that it makes invisibility irrelevant. The 5th edition description of the invisibility status say “An invisible creature is impossible to see without the aid of magic or a special sense. For the purpose of hiding, the creature is heavily obscured.” Blindsight certainly is a special sense. And when my ruling for ignoring obscurement is applied, it definitely allows the normal perception of invisible creatures and objects.

The minor illusion and silent image spells allow you to create an image. I would rule that a creature that can’t see the image with vision (normal or darkvision) would not even be aware of its presence. A creature that has both vision and blindsight would see it only with its regular vision, but not its blindsight. So I would rule that such a creature would automatically make Intelligence (Investigation) check to identify it as an illusion without having to use an action to examine it. In contrast, the major image spell creates an image that includes the right sound, smell, and temperature of the illusionary object, so I would rule that this fools creatures with blindsight just as well as any other creatures. I’d make the same distinction with the hallucinatory terrain and mirage arcane spells.

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