[RPG] What happens if you use the Magic Jar spell to possess a creature which has Spellcasting

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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.

Best Answer

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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