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According to Volo's Guide to Monsters, the elder brain and ulitharid have the following ability, with the main difference between the two being the effective range (2 miles for the ulitharid, 5 for the elder brain):

Creature Sense. The ulitharid is aware of the presence of crea­tures within 2 miles of it that have an Intelligence score of 4 or higher. It knows the distance and direction to each creature, as well as each creature's Intelligence score, but can't sense anything else about it. A creature protected by a mind blank spell, a nondetection spell, or similar magic can't be perceived in this manner.

However, if a ulitharid can know the exact distance and direction of an intelligent creature at all times, so long as they're within range and not under the effects of mind blank or nondetection, does that mean that, according to the rules as written, they are partially immune to the invisibility spell and to penalties imposed when fighting unseen attackers? After all, unless the proper protective spells are cast, the ability description seems to indicate the ulitharid always knows exactly where the attacker is, whether they're visible, invisible or hiding.

I don't think they'd be immune to being surprised, should they be attacked while sleeping or if they simply didn't expect the source of intelligence approaching to actually be the ranger instead of another ilithid, but I have my doubts regarding how the rules of hiding and invisibility should apply to such monsters.

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Knowing somebody's location does not mean you can see them

The rules on Unseen Attackers and Targets state (emphasis mine):

Combatants often try to escape their foes' notice by hiding, casting the invisibility spell, or lurking in darkness.

When you attack a target that you can't see, you have disadvantage on the attack roll. This is true whether you're guessing the target's location or you're targeting a creature you can hear but not see. [...]

When a creature can't see you, you have advantage on attack rolls against it. If you are hidden--both unseen and unheard--when you make an attack, you give away your location when the attack hits or misses.

So even when a creature casts invisibility on themself you still know where they are because they are not hidden, but merely invisible. However, because they are invisible, you cannot see them (without a special sense like from see invisibility).

Creature Sense does not let a creature actually perceive invisible creatures, instead it tells them their location. Compare this with blindsight which does entail perceiving your surroundings:

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


An incredibly lengthy aside on senses and "seeing"

Blindsight is seeing

The rules-text supports that Blindsight counts as seeing, but we also have some questions from the site to support this, in particular the one below and a certain part of the SAC:

Blindsight allows a creature to see the unseen, being invisible is bypassed by that sense and thus the mechanics that the Invisible condition provides are not activated.

An opportunity attack is triggered by “a hostile creature you can see”. If you can’t see an enemy, you can’t make an opportunity attack against it. Creatures with blindsight are an exception to this rule, because that ability lets those creatures “see” within a certain radius.


Blindsense is not seeing

Meanwhile we can also look at the Rogue's Blindsense:

If you are able to hear, you are aware of the location of any hidden or invisible creature within 10 feet of you.

Blindsense does not count as "seeing". I will support this with links to two other questions on this site and then quotes from their answers:

Knowing a creature's location means you don't have to guess where it is, but if you can't actually see it that still has consequences; you can't target it with spells that specify a target you can see, you have disadvantage on attacks against it, that kind of thing.

Blindsense doesn't mean you can see invisible opponents; it does mean that you don't need to guess at their locations.

Blindsense is very similar to Creature Sense, so treating them similarly makes sense here.


Tremorsense is probably not seeing

Unfortunately, there is also 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.

So does this count as "seeing"? It is certainly similar to Creature Sense and the rogue's blindsense, but the questions on this site seem to point a different way:

So my recommendation for a DM is this: make a ruling on how tremorsense works in your game. There are 2 logical rulings that I can see based on the description:

  1. Tremorsense is effectively a substitute for vision for targets that it can detect.
  2. Tremorsense allows the monster to know the precise location of a source of vibrations and little else.

Tremorsense and Blindsight are 'seeing' [...]

So while blindsight and tremorsense are not sight per se, within the realm of abilities that require sight, they can function as sight.

Tremorsense and Blindsight are not "seeing"

Tremorsense does not provide "sight" [...]

If tremorsense were intended to provide sight it would say so and would be more aptly names tremorsight, or tremorvision in line with blindsight, truesight, and darkvision.

So maybe the jury is out on Tremorsense, but it does seem like there is a consensus toward it not counting as sight as far as the votes and number of answers I've been able to find are concerned. It is also similar to Creature Sense (and Blindsense) so I would try to treat them all similarly, that is, that none of them actually count as seeing.

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  • \$\begingroup\$ A good point of comparison would be the blindsense feature from the rogue class, which provides similar locational information. \$\endgroup\$ Aug 21 '21 at 16:34
  • \$\begingroup\$ @RyanC.Thompson I thought about doing that, but it seems like people find it unclear how all these other senses really work when features require things to be seen. Including you, it would seem. But I can cherry-pick the answers that support my own, so I'll probably do that \$\endgroup\$ Aug 21 '21 at 16:36
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    \$\begingroup\$ The wording on a lot of these maybe-sight-replacing senses can be tricky, but I've always felt Blindsense in particular was pretty unambiguous. (And the fact that Blindsight already exists is a hint that Blindsense means something different.) \$\endgroup\$ Aug 21 '21 at 18:11
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They can get the location right, but attacks will still be made at disadvantage, and attacks against will still have advantage.

The rules for unseen attackers and targets are concerned with whether or not a creature is seen, not if its location is known:

When you attack a target that you can't see, you have disadvantage on the attack roll. This is true whether you're guessing the target's location or you're targeting a creature you can hear but not see. If the target isn't in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target's location correctly.

When a creature can't see you, you have advantage on attack rolls against it.

These rules don't really interact with the Creature Sense feature - all it does is tell you where a creature is, but these rules are concerned with creatures you cannot see, and even state that you still have disadvantage if you know where a creature is but cannot see it.

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