Undead are not "humanoid", they are "undead." That's an obstacle for Magic Jar.
I'm not sure when this would be useful.... but with the assistance of Simulacrum, what you(s) could do with some complicated set up is:
Cast Magic Jar, leaving your "living" body vacant.
Have you Simulacrum kill that body, turning it from a catatonic soulless creature into a corpse object.
Have you Simulacrum cast True Polymorph on that corpse object, to turn it into any medium sized creature CR 9 or below. Hint, pick one with great physical ability scores!
Re-inhabit the body using your special Magic Jar action, assuming that CR 9 creature's statistics and abilities, but keeping your own Intelligence, Wisdom, Charisma, and class features.
This ramps up your own abilities, but the jar becomes a double edged sword: on the plus side, if you keep it with you, you've got a chance to return to it in the future if your body dies (which would allow you to perform the same steps above, but you'd start over with a new CR 9 creature's statistics, you wouldn't keep the first creature's abilities you hijacked to build cumulatively). On the minus side, if the jar is destroyed, the spell will try to return you to your original body (which you're not really in, though kind of?), check whether your body is dead (and arguably it is?), and kill you instantly.
That can be somewhat mitigated by daisy chaining Clone with Magic Jar self-sacrifices, cloning yourself as the new possessed creature should create a "checkpoint" to let you die from Jar 1, but wake up in Clone 1 with your Jar 1 statistics saved. You can then create a Jar 2 interaction, but your Jar 1 creature abilities you inherited still aren't "class features," so they won't be carrying over with you into your Jar 2 body.
Can I use the magic jar to posses the undead I have created? Like, I can give my undead a gem and I posses them making people believe it's me
Magic Jar only works on humanoids and the kinds of undead a PC wizard can control generally don't pass as living humanoids anyways.
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Undead are not "humanoid", they are "undead." That's an obstacle for Magic Jar.
I'm not sure when this would be useful.... but with the assistance of Simulacrum, what you(s) could do with some complicated set up is:
This ramps up your own abilities, but the jar becomes a double edged sword: on the plus side, if you keep it with you, you've got a chance to return to it in the future if your body dies (which would allow you to perform the same steps above, but you'd start over with a new CR 9 creature's statistics, you wouldn't keep the first creature's abilities you hijacked to build cumulatively). On the minus side, if the jar is destroyed, the spell will try to return you to your original body (which you're not really in, though kind of?), check whether your body is dead (and arguably it is?), and kill you instantly.
That can be somewhat mitigated by daisy chaining Clone with Magic Jar self-sacrifices, cloning yourself as the new possessed creature should create a "checkpoint" to let you die from Jar 1, but wake up in Clone 1 with your Jar 1 statistics saved. You can then create a Jar 2 interaction, but your Jar 1 creature abilities you inherited still aren't "class features," so they won't be carrying over with you into your Jar 2 body.
Complicated.
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I'm going to make this way harder than it needs to be.