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Bats and Whales, and some monsters like the Hook Horror, have blindsight based on hearing. You can tell it's based on hearing because they also have the Echolocation trait that says their blindsight does not work if they are deafened. For the purpose of this discussion, please assume a totally dark environment.

Beyond the range of their blindsight, these creatures can use their mundane hearing to detect other creatures, and they can benefit from Keen Hearing, giving them advantage on Perception checks that rely on hearing. Within range of their blindsight, what changes? Do they still benefit from Keen Hearing? And how does blindsight change what they can perceive over and above mundane hearing?

In particular, to what extent can they pinpoint the exact position of creatures within and outside the range of their blindsight?

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  • \$\begingroup\$ Bats and whales can't target outside their blindsight range as they have no ranged attacks \$\endgroup\$ – Dale M Nov 14 '16 at 20:20
  • \$\begingroup\$ @DaleM Good catch. What I really wanted to know was whether they would be able to pinpoint a creature's exact location. I'll edit my question. \$\endgroup\$ – Clearly Toughpick Nov 14 '16 at 21:09
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RAW, Keen Hearing and Blindsight do not in any way work together or "stack". They are both used completely independently of each other. It does not require there to be "a completely dark room" to interpret the rules:

  • Within the range of the blindsight the creature can use blindsight to "see" and and can use wis(per) rolls with advantage from the Keen Hearing just the same as any other creature with Keen Hearing (e.g. attempting to hear whispers well enough to understand what is being said)
  • Outside the range of the blindsight the creature has no benefit from blindsight and can use wis(per) rolls with advantage just the same as any other creature with Keen Hearing (e.g. attempting to hear if anything is stealthily approaching)

The other senses of the creature operate as usual for the circumstances.

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    \$\begingroup\$ I was going to argue with your assertion that Keen Hearing and Blindsight don't stack. But thinking about it, they are two rather different things: if a creature's blindsight is of the echolocation type, then it is producing sounds and using their echoes to build a picture of its environment. This is sufficiently distinct from normal hearing that they can be viewed in isolation from each other. So you've changed my mind! \$\endgroup\$ – Clearly Toughpick Nov 14 '16 at 21:35
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    \$\begingroup\$ But this still leaves the question posed by @Sesdun's answer: how well can you pinpoint the location of a creature using just your hearing, whether keen or not (assume you have beaten its Stealth check)? \$\endgroup\$ – Clearly Toughpick Nov 14 '16 at 21:43
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    \$\begingroup\$ @ClearlyToughpick if you beat their stealth check (and disadvantage would not be inappropriate if you are blinded) then you know where they are. How this plays out narratively is up to you. If you detect them with "normal" hearing perhaps they broke a stick, if the keen hearing is why perhaps you hear their heart beating. \$\endgroup\$ – Dale M Nov 14 '16 at 21:58
  • \$\begingroup\$ I think Protonflux's response best answers the original question, but the other two responses, and the subsequent discussion, have been very useful too. \$\endgroup\$ – Clearly Toughpick Nov 16 '16 at 13:30
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Within the Blindsight: They act as normal, as if they see the target. If the target is blinded (by the darkness) they get advantage on their attacks.

Outside the Blindsight: They are Blinded.

  • A blinded creature can't see and automatically fails any ability check that requires sight.
  • Attack rolls against the creature have advantage, and the creature's attack rolls have disadvantage.

This however depends on the creature pinpointing the correct position and as far as I know there is no clear rules on how to do that - although a Wisdom(Perception) roll with advantage from Keen Hearing seems reasonable against a suitable difficulty.

It is also debatable if the creature grant advantage against projectiles as they need to pass through its blindsight radius before striking, but it seems RAW this is the case.

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  • \$\begingroup\$ @ Sesdun - I think your penultimate paragraph really gets to the heart of the matter. If you can't see, how well can you pinpoint the exact position of something by using your hearing? (I prefer to use the word position because I think it implies more precision than location.) \$\endgroup\$ – Clearly Toughpick Nov 14 '16 at 21:15
  • \$\begingroup\$ You are right, I will edit the post. To be honest I used location as a substitute for square. Position is better in a 5e /possible theatre of the mind -context. \$\endgroup\$ – Sesdun Nov 15 '16 at 13:00
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If the creature they want to target is outside their blindsight range and not hidden, they can target them with disadvantage from being blinded. This is not something particular to Keen Hearing, anyone can do this.

You appear to be making a mistake in thinking that if a creature cannot be seen then the default state is their location is unknown - in fact the default is the opposite: the location of a creature is known unless they are hiding. See What advantages does hiding have?

If they are hiding then Keen Hearing allows advantage on the Wisdom (Perception) check to beat their Dexterity (Stealth) check. Normally, this is passive versus active so advantage gives a +5 but if the perceiver wants to spend an action they can make an active check.

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  • \$\begingroup\$ '...the location of a creature is known unless they are hiding' is something I was hoping to flush out. Can you give a reference to a rule that states this please. I happen to think you have RAI right: - PHB p 291: Conditions - Invisibility, and - PHB p 194: Unseen Attackers and Targets both suggest to me that if you can hear a target, you can detect its location. And I think that means pinpointing its exact position, or five foot square on a grid. \$\endgroup\$ – Clearly Toughpick Nov 14 '16 at 21:53

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