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Magic Jar lets you possess a creature, potentially permanently. Spell text is:

Your game statistics are replaced by the statistics of the creature though you retain your alignment and your Intelligence, Wisdom, and Charisma scores. You retain the benefit of your own class features. If the target has any class levels, you can't use any of its class features.

If I were to take control of a Githyanki Knight, how would hit points, armor proficiencies, or weapons work? Would I get its hit points, and then add the hit points from my class on top of that? If it wears heavy armor, do I know how to wear the armor? And most oddly, in the case of a Githyanki Knight, do I have the ability to use its +3 silver greatsword?

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According to the Monster Manual p. 6-11 a creatures statistics consist of

  1. Size
  2. Type
  3. Tags
  4. Alignment
  5. Armor Class
  6. Hit Points
  7. Speed
  8. Ability Scores
  9. Saving Throws
  10. Skills
  11. Vulnerabilities, Resistances, and Immunities
  12. Senses
  13. Languages
  14. Challenge
  15. Special Traits
  16. Actions
  17. Reactions
  18. Limited Usage
  19. Equipment

Since Magic Jar states your statistics are replaced by the creatures statistics you get all of these except the handful that were excluded. You do not receive class features from the target creature however the Githyanki Knight has no class levels so this doesn't come up. You do receive the benefits of any class features (Everything in the class description following the header, "Class Features") you had. So then how does this work?

  • Hit points are part of the statistics so you use the Githyanki Knight's 14d8+28. On top of that you receive your hit dice (say 11d6 for an 11th level wizard). An interesting note here is that the hit points per level class feature also includes your constitution bonus. So in that case your total HP would be 14d8+28+11d6+22
  • Armor and Weapon Proficiencies aren't part of the creature's statistics so you retain your own proficiencies. Even if they were I'd argue that you would retain these since they are granted by your class levels. This ends up being less important though since...
  • AC is part of the creature's statistics so your AC is replaced with the Githyanki Knight's. Assuming that this is a normal knight wearing its plate you get an AC of 18.
  • Actions are part of the statistics so your normal actions are replaced. You now have a Silver Greatsword attack and a Multiattack. You still have other actions available since they are granted by your class levels.
  • Saving throws are part of statistics so they replace yours. However, you still retain the benefits of your class features so you are also proficient in any saves you had previously.
  • Innate Spellcasting is part of Special Traits so you get the psionic abilities.
  • Languages are part of the statistics so you now speak Gith. RAW, that's the only language you speak as languages are granted by race not class. Personally I'd rule that you still have access to languages you knew previously however I can't find anything in the books supporting that. Interestingly a Druid would still know Druidic since it's a class feature.
  • Your Size, Type, Speed, and so on are also all replaced by the Githyanki Knight's and then modified by any class features you have. Functionally you become a Githyanki Knight that has a bunch of class levels thrown on top.
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  • \$\begingroup\$ @harlandski good point, I've added a note clarifying that class features are everything in the class following the class features heading. \$\endgroup\$ – Ceribia Apr 20 '15 at 4:13
  • \$\begingroup\$ for hp, does that mean you get 14d8+28+11d6 ? \$\endgroup\$ – ohmusama Apr 20 '15 at 18:30
  • \$\begingroup\$ @ohmusama I've added a note about this. Since the class feature that gives you hit points for each level also adds your constitution modifier the example would be 14d8+28 (for the Gith part) +11d6+22 (for your 11 Wizard levels). It's a bit similar to how monstrous hit dice worked in 3.5. \$\endgroup\$ – Ceribia Apr 20 '15 at 18:36
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When read in context this is self-evident (to me anyway)

Once you possess a creature’s body, you control it. Your game statistics are replaced by the statistics of the creature though you retain your alignment and your Intelligence, Wisdom, and Charisma scores. You retain the benefit of your own class features. If the target has any class levels, you can't use any of its class features.

You are in control of their body - the "Your game statistics ..." means the statistics of you in the body you are controlling (your body at the moment is a hunk of meat - their mind/soul is in the gem). Every statistic is that of their body except for what the spell says are yours so from the top:

  1. Hit points - theirs
  2. Armour Proficiencies are a class feature so - yours
  3. Ditto weapon proficiencies
  4. You get their hit points - your body (wherever it is) has your hit points and can be alive or dead
  5. If you are proficient in heavy armour in your own body then you are in theirs - if not then ... not
  6. Silver Swords - proficiency depends on your proficiency. As to getting the psychic damage and +3 bonus -

Silver Swords. In ancient times, gith knights created special weapons to combat their mind flayer masters. These silver swords channel the force of the wielder's will, dealing psychic as well as physical damage. A githyanki can't become a knight until it masters the singular discipline needed to will such a blade into existence. A silver sword is equivalent to a greatsword, and takes on the properties of a +3 greatsword in the hands of its creator.

It "... channel the force of the wielder's will ..." and "... takes on the properties of a +3 greatsword in the hands of its creator." The will involved here would be yours; does this allow you to deal the psychic damage? That would be a DM call - I am in two minds about it. The hands involved are physically the githyanki's but does it mean literal hands or metaphorical hands? That would be a DM call - I am in two minds about this too.

On thinking it over, I would rule that you do not get either benefit. My decision is based on considering the opposite situation of the githyanki using magic jar on your body - to my mind it can use its will in your body to activate all the properties of its sword. So if it can, you can't.

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  • \$\begingroup\$ I disagree about Hit Points - they are the first thing listed under "Class Features" for every class. Also the description doesn't say that you don't get proficiencies from monster stat blocks, only that you don't get class features, but as monsters don't have class features in 5e, their proficiences are part of their 'statistics'. \$\endgroup\$ – harlandski Apr 20 '15 at 3:57
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The other answers do a fine job of discussing the general rules of what constitutes a creature's "statistics" and how these would replace the player's statistics. I'm particularly fond of the ruling Dale M. suggests for the Githyanki's Knight's abilities. That said, I now see that the Dungeon Master's Guide contains a section on page 283 regarding monsters with classes. It provides a ruling that sets Hit Dice apart as an exception to normal statistics when dealing with monsters who have class levels.

For each class level you add, the monster gains one Hit Die of its normal type (based on its size), ignoring the class's Hit Die progression.

Let's examine what this would mean given an 11th level Wizard with a +2 Constitution modifier possessing the Githyanki Knight. The Githyanki Knight's base Hit Points are 14d8 + 28 (the 28 comes from its Constitution modifier of +2, multiplied by its number of Hit Dice).

  • ~162hp or 25d8 + 50 with the DMG's Monsters with Classes Method
  • ~151hp or 14d8 + 28 + 11d6 + 22 if both the Class and Base Creature added together

Unfortunately this turns out to favor the Wizard even more, which I was already concerned about. I'm not certain that the DMG's rules should apply to a player possessing a creature - but its certainly valid rules-as-written.

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