D&D 5E Which abilities can detect a humanoid which is possessed by a ghost?

[Lackhand]

I'm planning ahead a little in my campaign, and am probably about to have a few villains who use living NPCs as catspaws and spies. They pick a living innocuous humanoid (a washerwoman, an orphan, a traveler, a beggar), put a loyal ghost in their head, and then the ghost comes back later with new information -- and usually brings the ridden NPC back, too, never to be seen alive again.
I know how I want to describe these creatures and their interactions, I know how I want to resolve using Wisdom (Insight) and roleplaying to uncover them, but I want to make sure I get the high-tech no-chance magical interactions right, too.

So: how does ghostly possession interact with various divinatory abilities, RAW and/or RAI (... and or RAF )?

From the SRD, some relevant rules text:

Detect Evil and Good
1st-level divination

Casting Time: 1 action
Range: Self
Components: V, S
Duration: Concentration, up to 10 minutes

For the duration, you know if there is an aberration, celestial, elemental, fey, fiend, or undead within 30 feet of you, as well as where the creature is located. Similarly, you know if there is a place or object within 30 feet of you that has been magically consecrated or desecrated.
The spell can penetrate most barriers, but it is blocked by 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt.

And, for instance, the Ghost's possession ability:

Possession (Recharge 6). One humanoid that the ghost can see within 5 feet of it must succeed on a DC 13 Charisma saving throw or be possessed by the ghost; the ghost then disappears, and the target is incapacitated and loses control of its body. The ghost now controls the body but doesn't deprive the target of awareness. The ghost can't be targeted by any attack, spell, or other effect, except ones that turn undead, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunity to being charmed and frightened. It otherwise uses the possessed target's statistics, but doesn't gain access to the target's knowledge, class features, or proficiencies.
The possession lasts until the body drops to 0 hit points, the ghost ends it as a bonus action, or the ghost is turned or forced out by an effect like the dispel evil and good spell. When the possession ends, the ghost reappears in an unoccupied space within 5 feet of the body. The target is immune to this ghost's Possession for 24 hours after succeeding on the saving throw or after the possession ends.

My take is obviously ambivalent:

Because the ghost's possession ability causes the ghost to "disappear", you might rule it's not physically there for detect evil and good to find while it's possessing a humanoid.
Because the ghost's possession ability specifies the ghost can only be targeted by certain spells -- and doesn't include detection powers -- you could rule it's undetectable by detect evil and good while possessing a humanoid.

Contrariwise, you might decide that the ghost has to be located inside of the humanoid host (see the text on "forced out by an effect"), and the humanoid host isn't a thin sheet of lead -- and so detect evil and good can find it.
You might decide the ghost can't be detected by detect evil and good -- but that the possessed humanoid "bears magic" with respect to the detect magic spell! If you did that, would this be enchantment magic, or necromancy magic?

So: Let me know your thoughts!

 

[Cap'n Kobold]

Detect Evil and Good's target is the person on whom it is cast. I can't see a reason why it wouldn't detect a ghost that happens to be possessing a person. Of course the person on whom it was cast may well assume that the ghost's victim is an undead creature in disguise rather than a living being being possessed.

 

[Sunseeker]

Detect Evil and Good's target is the person on whom it is cast. I can't see a reason why it wouldn't detect a ghost that happens to be possessing a person. Of course the person on whom it was cast may well assume that the ghost's victim is an undead creature in disguise rather than a living being being possessed.

To be specific, Detect Good/Evil doesn't have targets, it's just a radar. It picks up anything that it names (aberration, celestial, elemental, fey, fiend, or undead) within that radius. Warning, Pet Peeve Ahead: What I think is odd about the spell is that it doesn't actually tell you if any of those things are actually good or evil, it just assumes they must be based on their creature type.

As a DM, "Detect Good/Evil" would detect the ghost. Even if the Ghost used it's normal ability to become invisible while not possessing a host Detect Good/Evil would still detect it, would still tell you its location, even if you couldn't see it.

 

[Fanaelialae]

While it isn't a detection ability per session, Turn Undead would allow you to indirectly detect a ghost, in that once it is cast out you can see it.

I believe a paladin's Divine Sense would also work.

 

[steeldragons]

Paladin's Divine Sense specifically calls out sensing undead. A ghost in someone, in the presence of a paladin using this feature, would certainly be noticed, though the exact nature of why the person is setting off the paladin's metaphysicial radar -I would say- might require some roleplaying.

"There is an evil palor/presence infesting this person."

Is it a demon? Is it a ghost? Are they a vampire? Even a celestial being in disguise? Who knows? How do you find out? But, for sure, THIS guy is pinging on my Divine Sense.

 

[Lackhand]

... I can't see a reason why it wouldn't detect a ghost that happens to be possessing a person...

Taking this as literal, the rules are interestingly unclear. Let me see if I can demonstrate the other reading better.

The text of possession says the ghost "disappears" and that it uses the possessed creature's statistics (with some specific substitutions). Statistics includes creature type. Disappears can't mean the plain english "becomes invisible" because they would have said that. So instead it could mean "ceases to be present as an independent entity", which makes me wonder whether it's really there. If it's not really there, it can't ping on detect, and if it could, it uses the humanoid type, not the undead type.

The ghost is still subject to dispel evil and good (at a minimum). But the break enchantment line there refers to a creature possessed by a ghost, not the ghost itself. So that's (very weak!) evidence that the ghost isn't really there.

(also, I just noticed the possessed creature is incapacitated. Nothing seems to un-incapacitate it. So all a joyriding ghost can do is move around, no actions? Jeez, that's rough!)

 

[Lackhand]

... A ghost in someone, in the presence of a paladin using this feature, would certainly be noticed...

Maybe! But the text of Divine Sense includes that the divine sense is blocked by total cover.

What stops the "disappeared" ghost that seems to be located "inside" the possessed humanoid from having total cover?
It might be that the possessed humanoid provides only partial cover -- but the rules imply that total cover is one way to be untargetable, and possession includes similar untargetable language, so that certainly feels very similar...

 

[Lackhand]

One more thought (triple posting... sorry...) -- how does the "disappeared" ghost interact with See Invisibility?

Since the ghost isn't (rules-y) invisible or ethereal at this point, my contrarian reading indicates it doesn't ping, but others might disagree!

But since the general opinion is that detect evil and good and divine sense ping on a possessing ghost, I'm guessing that people generally think see invisibility pings for it, too?

(Full disclosure: I'm just trying to figure out what the rules themselves are saying. I think it makes divination effects more powerful and interesting to work in this case, so they will at my table. But I'm not entirely sure whether they're intended to!)

 

[transtemporal]

I'm guessing that people generally think see invisibility pings for it, too?

Yes, but just think how cool it would be to scare the crap out of the PCs by describing a ghost undead creature attached to the possessed persons head.

 

[steeldragons]

Maybe! But the text of Divine Sense includes that the divine sense is blocked by total cover.

What stops the "disappeared" ghost that seems to be located "inside" the possessed humanoid from having total cover?

The paladin's esoteric metaphsyical magical "Divine Sense," that's what. There is something which is specifically described as being revealed by the feature. If we apply your attempted argument, basically, an invisible demon in the presence of a paladin using Divine Sense would not be revealed...because they're invisible. That is clearly not how the feature works.

It might be that the possessed humanoid provides only partial cover -- but the rules imply that total cover is one way to be untargetable, and possession includes similar untargetable language, so that certainly feels very similar...

It's D&D, so I suppose it "might be." That is certainly your call to make. I would say that's nonsense. But that would be in my game. If the possessed person walks around a corner or closes a door that they're on the other side on, so the paladin can no longer see them, then yeah. They -the source of the "ping"- are behind "total cover" so the paladin can't sense anything because they can't see anything to target with the ability. In front of the paladin, in their presence/line of sight, the ghost is going to set off the Divine Sense alarm, but again, that doesn't tell the paladin anything very specific about the person/situation.

Divine Sense does say the "type" of creature is revealed, but nothing specific about them...I guess undead "smell" different than demons, that "feel" different than celestials, or however you want to describe the sensation of the feature. So they would be able to say that something "undead-ly" is about/around/effecting this individual...but the fact it was a ghost possessing them would not necessarily immediately be revealed.