What exactly is one creature

[Atalius]

With Magic Jar what exactly can't I magic jar besides incorporeal and constructs? Several elementals were encountered today but I am not sure if I can magic jar or not?

[Pizza Lord]

Outsiders wrote:

Traits:

Unlike most living creatures, an outsider does not have a dual nature—its soul and body form one unit. When an outsider is slain, no soul is set loose. ...[bunch more stuff about raising outsiders]

Took me a bit, I was used to seeing the wording for this under Elementals in 3.5, but they moved elementals to outsider category and consolidated.

Outsiders have souls and bodies, they just aren't separate parts of the creature. Some people might read that as their souls not existing, for instance, they might read 'When an outsider is slain, no soul is released,' to mean there is no soul, but the prior wording disputes that, they aren't soulless, they have soul/body unity (as opposed to the duality that most creatures have).

That doesn't mean trapping the soul destroys the body. If it did, then an outsider using magic jar would have their body crumble to dust when they move into the gem receptable and I don't recall any pertinent adventures or statements about outsiders not being able to use the spell. Since this is identical to the reverse effect, ie. being stuffed into a gem by someone else using magic jar, I would have to conclude that you are able to take over their body and their life force/soul/personality/ego/etc. is trapped and their body remains behind with you in control just like any other creature's would. Obviously, if a certain outsider is specifically noted as being soulless or something like that, that would be a specific exception (you wouldn't even be able to target or sense them while in the gem).

Otherwise, you should be able to take over the body of any living creature (again, unless it has no 'soul' specifically, which outsiders do regardless of their lack of 'duality'), plants, oozes, demons. I believe the spell uses soul and life-force interchangeably for ease of reference and is basically implying that it's the creature's psyche and personality, but to keep the wording short on an already complex game mechanic and spell, they had to go with brevity and a simple assumption that the spell is expected to work in most cases. without naming every possible iteration of 'identity' that every religion, philosophy, or culture might have for 'the soul'.

[avr]

Swarms are also out of course, and personally I'd think twice about taking over a Lovecraftian aberration, but yes elementals should be on your target list as Pizza Lord explained.

[VRMH]

Pizza Lord wrote:

That doesn't mean trapping the soul destroys the body. If it did, then an outsider using magic jar would have their body crumble to dust when they move into the gem receptable and I don't recall any pertinent adventures or statements about outsiders not being able to use the spell.

I always thought that's precisely what happens: without the animating spirit, its physical embodiment simply ceases to be - only to reform once the soul has been released.

This interpretation would allow Outsiders to use Magic Jar but not be subjected to the spell as a target.

[blahpers]

An outsider using possession does in fact go all-in, so to speak--it acts like greater possession and their entire body goes into the host. I'd expect magic jar to work similarly, but there's certainly room for variation there.

[Pizza Lord]

If using possession as the foundation for possession effects (which I believe we should) that is further example that you can take over the bodies of incorporeal or non-native outsiders.

There is still suitable difference in their mechanics, however, as to trapping them (as in the gem, with magic jar, versus imprisoning their consciousness in their own body, with possession), so it's the GM call on that.

[blahpers]

Indeed.

I've long considered magic jar to be a good example of how increased verbosity can actually make a rules element less clear rather than more clear. Possession feels like someone said, "ugh, let's do this spell over so it makes sense".