So this is just casually the most op spell in dnd

Quote from Mannimarco_lord_of_evil_kittens >>

True you are not immortal in the traditional sense but if you are smart about it you can attain a form of pseodo-immortality.

for example if a wizard somehow manages to get enough gold and resources to produce or procure enough young bodies through the clone spell or kidnapping they could theoretically posess new body after new body for basically forever.

The only downside is that you somehow need to keep your original body alive but this can be achieved through the Imprisonment spell as well as somehow managing to maintain a constant supply of fresh young bodies.

 I think Sequester might be better than Imprisonment.

If your soul goes into your own body the spell ends. (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.)

Is there any way to end someone's possession by a non-undead humanoid from this spell? Every spell I've read so far (Protection from Evil and Good, Dispel Evil and Good, Magic Circle, Hallow) can only end possession against "celestials, elementals, fey, fiends, or undead."

The only thing I see is destroying the container, but if it is held somewhere far away, this quickly turns into a hostage situation as soon as it is realized.

How do hit points work do you add your own hit points to the original creatures, are the original creatures hit points replaced with yours, or are you stuck with the creatures hit points

"You retain the benefit of your own class features" would indicate that you would gain your hit points as they are also a class feature but I would like to know for certain before I take the spell

You would have their hit points now, not yours. That's explained in the previous sentence: "Your game statistics are replaced by the statistics of the creature, though you retain your alignment and your Intelligence, Wisdom, and Charisma scores."

My understanding is class features include things such as Spellcasting, Extra Attack, Necromancy Savant, etc. These are the things that on DnDBeyond you can look at in Features and Traits tab under Class Features specifically.  Statistics would be most other things.

Edit: Clarification in the first sentence.

If you anyways have that many clones why not make them of yourself and use clone instead?

Couldn’t you become statistically immortal like this?

unless they had dispell magic?

Quote from Cynnewulf >>

Quote from RonSnow >>

SO.... Can I:

A. Move my soul to a (let say) a mindflayer 100ft away...

B. The Mindflayers soul gets into the jar...

C. I slit my mindflayer throat as an action...

D. My companions smash the jar on their turn. I return to my body...

E. The mindflayers soul return to its bleeding out body and dies.

F. Profit ????

 Not exactly.  You can however, say...

A. Possess a guard of the evil king's castle

B. "Arrest" your fellow PC's to get them into the castle

C. Find a quiet place to have your party "deal with" your host body so you can upgrade to ...

D. Possess the king!  Appoint the rest of the party to high positions in your court.

E. You just won D&D.  ;-)

 now just use the money to make clones apiont you as next king die once clone reachers amturite (yay no need for a puppet)

Quote from CatYawn >>

So this is just casually the most op spell in dnd

 think clone is better

Quote from NavyDave >>

If you anyways have that many clones why not make them of yourself and use clone instead?

 yeay semms much better idea as you have your stats rather than who ever you are possesing

be 20 level wizard 

Get ruby of the war mage

cast magic jar using the ruby

cast imprisonment on body and chose the gem option using the ruby

find sword of Kas and attach ruby to it

kidnap a bunch of humanoids with good strength, dex, and con

profit 

Filthy jarman

Burn through a Great Wyrms legendary saves, have an ally polymorph it into a crippled blind man, magic jar into the "man", release the polymorph on the Great Wyrm, Wish that this is now your original body which will not react to anti magic or reversal effects.

Over CR 20 Great Wyrm with 17+ Mage levels. Thank your DM for being a creativity positive enabler.

Now go kill a god or something. 

One thing to note from this spell:

If you're a barbarian, you probably have a very high Strength score and at least an average Constitution and Dexterity. Then, when your party just reached level 11, watch out for your wizard teammate.

I mean, the way I read it, you get the target's hit points, right? Anyone willing to trade D6 for D12, and to get an increase in Strength? You could also probably end up improving your Con and Dex and maybe even getting Relentless Endurance if you target a half-orc, although this may be up to interpretation by the DM as to whether this trait is part of the creature's statistics.

Either way, you can upgrade your body in a minute. You'll just need to make sure that the container is hidden somewhere safe, or keep it on you and retreat back into it just before dying.

Very important question about this spell: do you retain your proficiency bonus, or do you use the proficiency bonus of the creature you turned into?

If no, then using this spell essentially nerfs your own spell save DC

If yes, then you could have a lot of fun in possessing a sahuagin baron and casting tenser's transformation 

So it seems like the spell will not end if the reliquary/soul is more than 100 feet away from the original body. So in theory, this could be a way to trap your soul for eternity if the soul box is moved away from your body, you have no humanoids to possess and the box is never destroyed. In theory, you could use suggestion on a powerful wizard to make them cast this spell, then take the box and lock it away in another box to eliminate line of sight and bam! infinite soul prison. Quite an awful fate.

I think this spell is a little too powerful if it allows someone to permanently discard their own body and steal someone else's. Here are two (non-exclusive) ways I think of homebrewing it.

"If the spell's duration reach 24 hours, and every 24 hours after that, roll a D6. A result of 1-2 reduce your Intelligence score by one, a 3-4 reduce your Wisdom score by one, and a 5-6 reduces your Charisma score by one. Every ability or feature using these scores is affected, including your spell DC for the current Magic Jar spell. These reductions cease when the spell ends."

"If your original body dies, the spell ends and you die."

Very important question about this spell: do you retain your proficiency bonus, or do you use the proficiency bonus of the creature you turned into?

The proficiency bonus is part of your class features, so you retain yours.

As for proficiencies, I would probably follow the same rules as a druid's wildshape ability: "You also retain all of your skill and saving throw proficiencies, in addition to gaining those of the creature. If the creature has the same proficiency as you and the bonus in its stat block is higher than yours, use the creature’s bonus instead of yours. If the creature has any legendary or lair actions, you can’t use them."