Can a simulacrum cast Magic jar?

Question

Suppose you are a High level Wizard with acess to both simulacrum and magic jar. There is a lot of discussion if the simulacrum has or doesn't have a soul, and if that interferes in the use of magic jar, since the spell specifically describes the trasportation of the soul.

The spells says:

Your body falls into a catatonic state as your soul leaves it and enters the container you used for the spell's material component. While your soul inhabits the container, you are aware of your surroundings as if you were in the container's space. You can't move or use reactions. The only action you can take is to project your soul up to 100 feet out of the container, either returning to your living body (and ending the spell) or attempting to possess a humanoids body.

That means that it does not describe the need of you to be a humanoid to cast.

So, is there anything in the RAW that prevents the simulacrum from casting magic jar except for lore purposes?

Answer 1

RAW, nothing prevents a simulacrum from casting magic jar.

This answer details the ambiguity behind D&D 5e's use of the word soul and demonstrates that the best we can do with the question "does a simulacrum have a soul?" is to say "Maybe, it's a DM judgment call."

That being said, there is nothing mechanically that would prevent a simulacrum with a 6th level slot from casting magic jar on itself. You have correctly observed that the only target mentioned is "self" with no creature type limitations. If a DM determines that a simulacrum has no soul and that a soul is required to cast magic Jar, that would not be an unreasonable ruling, but permitting a simulacrum to cast it seems just as reasonable.

It's up to the DM.

Answer 2

Your body falls into a catatonic state as your soul leaves it and enters the container you used for the spell's material component.

Alternative 1: Simulacrum spell does not say it does not have a soul, so it must have a soul. Magic Jar works as usual.

Alternative 2: Magic Jar says the caster's soul transfers, so it happens. It doesn't matter if caster has a soul or not, because magic.

Alternative 3: Simulacrum does not have a soul, because spell does not say it has. But Simulacrum will still cast the spell successfully, transferring the closest thing to a soul it has into the Jar: the soul of the original caster. The spell proceeds as normal, as if the original caster had cast it.

Alternative 4: Simulacrum has no soul. Casting Magic Jar transfers no soul to the Jar. There is no soul in the jar, so rest of the Magic Jar description is irrelevant.

The DM has to choose, RAW does not really help here.

Answer 3

RAW is unclear

Originally, simulacrum did not specify a change in creature type (so a sim of a humanoid would still be humanoid); however, it was errata'd to state:

Otherwise, the illusion uses all the statistics of the creature it duplicates, except that it is a construct.

So the real question is: does a construct have a soul? The introduction to the Monster Manual (p. 6) has this to say about constructs:

Constructs are made, not born. Some are programmed by their creators to follow a simple set of instructions, while others are imbued with sentience and capable of independent thought. Golems are the iconic constructs. Many creatures native to the outer plane of Mechanus, such as modrons, are constructs shaped from the raw material of the plane by the will of more powerful creatures.

Even here, it's not clear if constructs have souls. On one hand, some are merely programmed by their creators. On the other hand, some are "imbued with sentience". Neither gives a definitive answer.

Different spells and effects imply different things

Yes

Raise dead only works if "the creature's soul is both willing and at liberty to rejoin the body". It explicitly "can't return an undead creature to life", but provides no restriction for constructs. Resurrection the same restrictions.

No

Nine lives stealer doesn't work on undead or constructs:

If you score a critical hit against a creature that has fewer than 100 hit points, it must succeed on a DC 15 Constitution saving throw or be slain instantly as the sword tears its life force from its body (a construct or an undead is immune).

This may imply that they don't have a life force to tear from their body. The legendary weapon Blackrazor (DMG, p. 216) has the following similar feature:

Devour Soul. Whenever you use it to reduce a creature to 0 hit points, the sword slays the creature and devours its soul, unless it is a construct or an undead.

A banshee's Detect Life cannot sense undead or constructs:

Detect Life. The banshee can magically sense the presence of creatures up to 5 miles away that aren’t undead or constructs.

The banshee's Horrifying Visage does work against constructs (though undead are still immune), so there may be something special about the (lack of) life in a construct.

Conclusion

The first time this came up, I would probably allow a sim to cast magic jar normally, but remind the player that they're entering a gray area. I'd warn them that I reserve the right to re-evaluate my ruling if the sim + magic jar combo becomes problematic in the future.

There's plenty of ammo to rule either way, but the end result is something you and your group will have to discuss.