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I would like to know what happens if you use the Magic Jar spell to possess a creature which is capable of casting spells. Below are 3 different situations. I post all these questions in the same question because all of them are heavily related.

1st Situation: Creatures with Innate Spellcasting.

For example Kraken Priest (MM 215).

Would I be able to use its Innate Spellcasting? I mean, being able to cast its spells as the creature does (X uses per day). I don't say using my spell slots because that is impossible (eg. a Drow Elf Warlock can't use its spell slots to cast Darkness).

2nd Situation: Creatures with Spellcasting (similar to Pact Magic).

For Example Warlock of the Archfey (MM 219).

Would I be able to use its spellcasting? Note that this creature is an "11th-level spellcaster", not an "11th-level warlock" (its spellcasting isn't a class feature).

What would happen with the spell slots? Would I be able to cast its spells with my own slots? Would I be able to cast my spells with its slots?

3rd Situation: Creatures With Spellcasting (similar to Spellcasting of Clerics or Wizards)

For Example Diviner (MM 213).

Would I be able to use its spellcasting? Note that this creature is a "15th-level spellcaster", not a "15th-level wizard" (its spellcasting isn't a class feature).

What would happen with the spell slots (considering that for example, I'm a 16th-level Wizard)?

  1. Would our spellcaster level sum (as if multiclassing)?
  2. Should I use the highest spellcaster level instead of summing them?
  3. Should I count the spellcaster level separated, each one with its spell slots separated? For example, Would I have 8 1st-level spell slots (4 + 4)?

Finally (without considering a specific situation):

Would I have to recalculate the spell's attack bonus and spell's DC if my mental stats are different from the creature's stats?

I know that these are a lot of multiple questions, but I don't think that posting them separately would be beneficial. If you think so, let me know and I will do that.

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1. Creatures with Innate Spellcasting

From the description of Innate Spellcasting in the Monster Manual (p. 10):

A monster with the innate ability to cast spells has the Innate Spellcasting special trait. Unless noted otherwise, an innate spell of 1st level or higher is always cast at its lowest possible level and can't be cast at a higher level. If a monster has a cantrip where its level matters and no level is given, use the monster's challenge rating.

An innate spell can have special rules or restrictions. For example, a drow mage can innately cast the levitate spell, but the spell has a "self only" restriction, which means that the spell affects only the drow mage.

A monster's innate spells can't be swapped out with other spells. If a monster's innate spells don't require attack rolls, no attack bonus is given for them.

These are not "class features", so you can use them.

2. Creatures with Spellcasting (similar to Pact Magic) & 3. Creatures With Spellcasting (similar to Spellcasting of Clerics or Wizards)

From the description of Spellcasting in the Monster Manual (p. 10):

A monster with the Spellcasting class feature has a spellcaster level and spell slots, which it uses to cast its spells of 1st level and higher (as explained in the Player's Handbook). The spellcaster level is also used for any cantrips included in the feature.

The monster has a list of spells known or prepared from a particular class. The list might also include spells from a feature in that class, such as the Divine Domain feature of the cleric or the Druid Circle feature of the druid.

These are "class features" so you can't use them.

Finally (without considering a specific situation)

Yes, you would have to recalculate them. For example, the drow statblock says:

Innate Spellcasting. The drow's spellcasting ability is Charisma (spell save DC 11).

If your Charisma is different than 12 (in this instance), you need to change the DC.

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    \$\begingroup\$ 1) Magic Jar says "If the target has any class levels, you can’t use any of its class features." Since a Diviner has no class level, you have no restriction to take its class features, if any. 2) "The monster has a list of spells known or prepared from a particular class." The Diviner says that it is a "15th-level spellcaster". Even if it is evident that its spells are from the wizard spell list, it is not a Wizard (there could be others Intelligence full-casters with those spells). \$\endgroup\$ – Enderluck Feb 3 at 23:55
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    \$\begingroup\$ @Enderluck if you know the answer, post the answer \$\endgroup\$ – Dale M Feb 4 at 1:16
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    \$\begingroup\$ I don't know the answer. I was just replying your answer explaining why I don't agree with your opinion. \$\endgroup\$ – Enderluck Feb 4 at 1:21
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    \$\begingroup\$ @Enderluck then you must have your own opinion - we’d love to see it. \$\endgroup\$ – Dale M Feb 4 at 3:53
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The rules are slightly ambiguous in these cases, but it seems more logically sound to conclude that you would get spell-like abilities and inherent spellcasting, but would not get a spellcasting feature.

In order:

1st Situation: Creatures with Innate Spellcasting.

You should gain access to these abilities. Innate spellcasting does not work in the same manner as class feature-based spellcasting, and these are similar to any other ability in a stat block (such as natural weapons, inherent multi-attacks, and so on). There is no special knowledge, force of personality, or understanding which is required to produce them, and so the lack of the target's knowledge and mental stats doesn't apply.

2nd Situation: Creatures with Spellcasting (similar to Pact Magic).

The way that the Warlock of the Archfey is described is clearly similar to the Warlock class ability. It's explicitly described in Volo's Guide to Monsters as gaining its powers through a pact with a lord of the Feywild, and it gets a number and level of spell slots similar to a Warlock-classed creature, regains those slots on a short rest, and so on.

Generic stat blocks for NPCs don't usually include class levels, but it's difficult to argue that this creature's non-innate spellcasting is anything but Pact Magic, and therefore you don't get access to it. It's power coming from a patron, and Magic Jar isn't going to convince the patron through its mystical bond into thinking that you're the target any more than wearing a mask that looks like the target's face would.

You get to cast spells you already know through means you already have (you retain your mental stats and class features, including spellcasting through any avenue). Your spell slots do not change, nor do your spells known.

3rd Situation: Creatures With Spellcasting (similar to Spellcasting of Clerics or Wizards).

These magical effects aren't a product of what a creature is, it's a product of what they know (Wizard-like spellcasting) or the favor a deity grants them (Cleric-like spellcasting). In the former case, you don't gain the mental abilities nor the knowledge of the target, so even if the target knows spells you do not know them simply by dint of inhabiting the target's body. In the latter case, the deity favors the target, not you, and so they aren't just going to let you tap and channel their divine power just because you've taken over the body of someone they would grant that ability.

You don't get access to the target's spell slots in any way (directly, or summing them with your own), and you don't get to know the spells that they know. Your spellcasting ability is exactly what it was before the possession, because this type of spellcasting depends on attributes you specifically do not get from the possession.

And yes, you would need to recalculate spell attack bonuses and spell save DCs based on your own spellcasting ability. You are casting the spell, not the target you've possessed, and those values are based off of your mental stats, which remain your own and not your target's.


Rationale

As mentioned in the header on this answer, the rules as written are a bit ambiguous because PCs only get access to spellcasting via class features while NPC stat blocks generally do not include class levels even when they include spellcasting.

Due to the ambiguity we're stuck working by analogy. Magic Jar explicitly states that you get the physical stats of the body you're possessing (STR, CON, and DEX) and that you retain your original mental stats (INT, CHA, and WIS). This basic fact suggests a couple of questions:

1. What are innate spellcasting and spell-like abilities?

These are effects that are magical or are like magic, but don't require an underlying stat to produce. A Red Dragon gets to breath fire, and whether or not that's a product of manipulating the Weave in some way it's definitely not a spell. The dragon doesn't apply arcane formulae or verbal, somatic, or material components to make the effect happen, nor does it appeal to a deity to produce the effect, nor does it tap a mystical connection to a patron according to a Pact to produce the effect. No spell slots are expended, nor can they be, to produce the effect, whether or not the creature has them.

Such an effect is, then, a product of what the dragon is rather than what it knows, is granted by someone else, or is owed by someone else. This fits well with creatures that have poor mental stats, like a Phase Spider. They can use Ethereal Jaunt, but it's not knowledge, force of personality, or understanding which causes it to happen.

That strongly argues in favor of being able to use such abilities when possessing a creature which has them via Magic Jar.

2. What is casting a spell?

In contrast to (1), casting a spell (in the sense of consuming a spell slot) is a result of manipulating the Weave. Arcane casters do that through knowledge and ritual (understand the Weave, do things that cause it to be altered in some predictable way, produce an effect). Sorcerers' powers work similarly, but are built off of an intuitive understanding of the Weave rather than a formal, formulaic study of it.

Divine casters perform magic by tapping into divine power and channeling it in specific ways, with the ability to tap that power being granted by a deity's favor-- a god bends the world (through whatever specific mechanism) to produce a magical effect.

Pact magic works via an explicit agreement between an entity that can grant magical ability and someone else-- the power is provided by the patron and directed by their counterparty.

The common thread among all of these is that spellcasting is not related to who or what an entity is. It's about things they know, things they've done, and ongoing, mystical relationships with others. In such a case there is no reason to think that you would be able to inherit those after possessing a creature's body. In addition to explicitly retaining your own mental stats, there is no mention of gaining any of the knowledge or relationships of your target.

So if you possess an INT-based spellcaster, the more direct analogy is to a creature that has studied arcane magic and learned specific spells (like a Wizard) than to a creature that instinctively can produce a magic-like effect (like the Phase Spider's Ethereal Jaunt). Since you don't know what they know, you don't get the benefits of what they know.

Similarly, there is no reason to think that you could get any other kind of spellcasting feature through avenues. If you can't get access to Divine Spellcasting just by dressing like and imitating Clarence the Cleric, cramming your soul into another body should be similarly ineffective. If an Archfey has struck a pact with someone else, well, that pact isn't with you, and so you don't get to draw on any of the patron's powers.

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  • \$\begingroup\$ So, for the class-like spells, would you let magic jar give you Sorcerer-like spellcasting? Your argument against the current class list seems to imply that. \$\endgroup\$ – Please stop being evil Feb 4 at 22:17
  • \$\begingroup\$ @Pleasestopbeingevil No, that's not my meaning. A Sorcerer uses their CHA to manipulate the Weave, and settles into certain effects they know how to produce (that's their spell list), which expands with time and experience. If you got to use the target's CHA and knowledge/experience, then sure, why not? But Magic Jar doesn't do that, and even if you end up in a fundamentally magical, Sorcerous body you don't have the target's CHA or accumulated experience to draw on to produce effects. Maybe you could develop them over time spent in that body, but that's the same as taking class levels. \$\endgroup\$ – Upper_Case Feb 4 at 22:28
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    \$\begingroup\$ "Sorcerers carry a magical birthright conferred upon them by an exotic bloodline, some otherworldly influence, or exposure to unknown cosmic forces. One can’t study sorcery as one learns a language, any more than one can learn to live a legendary life. " \$\endgroup\$ – Please stop being evil Feb 4 at 22:34
  • \$\begingroup\$ @Pleasestopbeingevil But a level 0 Sorcerous creature knows 0 spells of any type. A Sorcerous creature with at least 1 class level in Sorcerer knows more spells than that, and the number of spells they know (and how flexibly they can perform magic) increases with Sorcerer level. It's not a matter of formal study, but Sorcerers clearly develop in power through experience, much like anyone can swing a sword, but only someone with sufficient class levels (or an appropriate entry in their stat block) gets to make two weapon attacks per turn. It's potential ability vs. realized ability. \$\endgroup\$ – Upper_Case Feb 4 at 22:41
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I'm going to disagree with the other two answers for cases 2 and 3.

On page 10, the Monster Manual says "spellcaster level", not "spellcaster class level". Later it says "considered a member of that class", not "has class levels in" or similar language, and that is only in reference to attuning to and using magic items.

Many creatures in the Monster Manual have abilities similar to even identical to class features (e.g. the spy's cunning action, of 349) that aren't called class features.

Rules as written, you are allowed to use spellcasting because it is not a class feature. Rules as intended, it seems that these creatures were intended to be treated as if they took levels in those classes without the unnecessary added complexity of actually having class levels and that is how they acquired the spellcasting feature. Therefore the apparent intention was to prevent using this feature, but they missed this particular situation.

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  • \$\begingroup\$ This answers part of the question, but doesn't address the spell slot portion (not a criticism, just a note that addressing it in the answer might be good). Additionally, we're trying to use the RAI acronym less often and more carefully, in case that affects how you would write the answer. \$\endgroup\$ – Upper_Case Feb 4 at 22:02
  • \$\begingroup\$ I've changed the wording to talk about spellcasting instead of spells. That should cover everything. \$\endgroup\$ – Johnathan Gross Feb 5 at 5:35

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