dnd-5espells
Is it possible to cast Magic Jar from the host body? If yes, is the ex host body treated as the starting body described in the spell?
Some scenarios I would like considered are:
What happens to the first container and first possessed body upon casting of magic jar the second time?
Does the possessed body count as "your body" for the subsequent spell castings?
If the 2nd host is killed, do you return to body of the first host? (in other words, does the spell daisy chain?)
The spell magic jar has a series of clauses making missteps extremely dangerous, both for the possessor and the possessed, but the beast-bonded witch's supernatural ability twin souls changes that effect with three parenthetical words. It says that
...if the witch or her familiar is gravely injured or about to die, the soul of the dying one immediately transfers to the other’s body. The two souls share the surviving body peaceably, can communicate freely, and both retain their ability to think and reason. The host may allow the guest soul to take over the body temporarily or reclaim it as a move action. They can persist in this state indefinitely, or the guest can return to its own body (if available) by touch, transfer into a suitable vessel (such as a clone), or take over another body as if using magic jar (with no receptacle).
Emphasis mine. First, for the ability to activate, the GM must determine that either the witch or the familiar is either gravely injured or about to die (which means, I assume, the creature has the condition dying but ask the GM).1
Assuming the witch activates the special ability twin souls by whittling himself to dying 1 point of damage at a time (the witch doing this to himself rather than to his familiar because the witch needs the familiar to prepare spells), the witch then inhabits the familiar's body as per the description of the ability twin souls and can use an effect like the spell magic jar "with no receptacle."2
What this parenthetical means is deeply unclear. This 2012 thread offers one interpretation. Below I present an alternative that combines the special ability twin souls and the spell magic jar stripped of all references to the receptacle. Such an ability reads...
You can attempt to take control of a nearby body, forcing its soul out of its body, killing it if it has nowhere to go. The spell ends when you send your soul back to your own body.
Attempting to possess a body is a full-round action. It is blocked by protection from evil or a similar ward. You possess the body and force the creature's soul out unless the subject succeeds on a Will save. Failure to take over the host leaves your life force in the current body, and the target automatically succeeds on further saving throws if you attempt to possess its body again.
If you are successful, your life force occupies the host body, and the host's life force is pushed out. You keep your Intelligence, Wisdom, Charisma, level, class, base attack bonus, base save bonuses, alignment, and mental abilities. The body retains its Strength, Dexterity, Constitution, hit points, natural abilities, and automatic abilities. A body with extra limbs does not allow you to make more attacks (or more advantageous two-weapon attacks) than normal. You can't choose to activate the body's extraordinary or supernatural abilities. The creature's spells and spell-like abilities do not stay with the body.
If the host body is slain, you die, and the life force of the host departs (it is dead). Any life force with nowhere to go is treated as slain.
If the spell ends while you are in a host, you return to your body (or die, if it is out of range of your current position).
Alternatively, you can take a move action to touch your body to return your life force to it.
...And while that seems crazy, it's not quite as crazy as it sounds. Yes, this magic jar effect is a save-or-die, but a beast-bound witch must be at least level 10 to use this ability, which means save-or-die effects have been available for at least 3 levels (e.g. phantasmal killer). Further, the witch had to render himself dying first to have access to the ability. Also, once the magic jar effect is used successfully, the only way the witch can use the ability again is by rendering the new body dying again, transferring to the familiar, and finding another body to possess; the ability doesn't appear to work while the witch is possessing a creature, but ask the DM. Finally, the duration of the magic jar effect remains: it's 1 hour/level, and the only longer-lasting switch the witch can make is to his own body or a "suitable vessel," a term the GM must also clearly define.
(Remember, too, that once the witch fails to possess an individual, the witch can never possess that individual. That might be a hassle.)
Officially, however, what the ability should do is mysterious and in the GM's hands. Talk with the GM first before attempting to exploit it. Also, I urge you to press the FAQ button on this post—as I did—to encourage Paizo's development team to address the beast-bound witch's twin souls ability in a future FAQ.
1 This GM would rule that an effect that straight-up, flat-out kills the witch or familiar still leaves the witch ding-dong dead, and the twin souls ability goes unused. Being gravely injured or about to die is a thing, but dead is dead. This GM doesn't view the the special ability twin souls as having a kind of limited omniscience, but another GM may. Such is a risky tack to take, however, possibly making a nigh-unkillable character, one that keeps his familiar in a secure yet comfortable lair miles (or continents or planes) away from any direct engagement, and who uses acquired bodies to depart the lair and harry his foe. This is, essentially, a dirt cheap (and likely murderous) variant of astral projection yet 7 levels early.
2 My gut says that with no receptacle should've said using the witch's or the familiar's body as a receptacle yet space constraints forbade that language. But that is totally speculation on my part and isn't what the ability says.
This is a difficult situation because there's no real specification of what exactly Death Ward covers when you target the "self". For all intents and purposes a creature is both simultaneously a body and a soul as a single unit, and only specifies each part when they are split.
My argument then, would be that Death Ward, when cast in this fashion, only exists so long as the soul and the new body are combined, as that is the "creature" the spell was cast on.
Order of operations:
Breaking the jar is not the trigger that causes instant death (even though the soul would immediately die) thus I would argue Death Ward does not activate. The soul then dies as it specifically was not protected by the spell.
If you specified that the spell was cast on the soul, it might postpone your death by 1 round as the soul continues to exist in some form of limbo but the end result will be the same.
This is debatable and will likely fall under DM jurisdiction if they consider the Soul to essentially always be the target of spells such as this - the soul is truly who the creature is. The Soul however cannot survive without a host.
Additional options (DM approval) could be some form of permanent limbo that requires outside help to escape, or continuing to exist in an undead state like a ghost or specter. But these options are not strictly RAW.
Best Answer
No, not according to RAW.
The spell description makes a clear distinction between your body (described respectively as 'your body' or 'your living body') and the possessed body (described as 'target's body' or 'host body'). Therefore, as specified in the spell, Magic Jar must be cast from your body not the host body.
The spell never mixes the two terms and distinguishes what you can do from your body, the host body, and the soul container. I would say you cannot chain the Magic Jar spell as suggested above. Sadly, many 5e spells appear to be weaker or less versatile than their older versions. If you would like to play around with this in 5e, it would be homebrew territory.