You attempt to beguile a creature that you can see within range. It must succeed on a Wisdom saving throw or be charmed by you for the duration. If you or creatures that are friendly to you are fighting it, it has advantage on the saving throw.
While the creature is charmed, you have a telepathic link with it as long as the two of you are on the same plane of existence. You can use this telepathic link to issue commands to the creature while you are conscious (no action required), which it does its best to obey. You can specify a simple and general course of action, such as "Attack that creature," "Run over there," or "Fetch that object." If the creature completes the order and doesn't receive further direction from you, it defends and preserves itself to the best of its ability.
You can use your action to take total and precise control of the target. Until the end of your next turn, the creature takes only the actions you choose, and doesn't do anything that you don't allow it to do. During this time, you can also cause the creature to use a reaction, but this requires you to use your own reaction as well.
Each time the target takes damage, it makes a new Wisdom saving throw against the spell. If the saving throw succeeds, the spell ends.
At Higher Levels. When you cast this spell with a 9th-level spell slot, the duration is concentration, up to 8 hours.
So... Can a monster who casts this innately, such as a Mind Flayer, can they use the lvl 9 version?
"If the DM says so" is the answer of course. But for the sake of argument, should they be able to?
I believe innate essentially means dont need spell components for the spells. A mind flayer has "1/day each: dominate monster, plane shift (self only)".
Based off of that, I would say a mind flayer can cast this spell as stated. Not at higher level though. If some low level challenge rating creature had 1/day: magic missiles, that means that they get to cast just magic missiles at level 1, that or else they would just cast it at 9th level every time and murder the whole party :P
I might let an advanced Mind Flayer use the 9th level version if there were something special about it--if it were an ulitharid for example. Otherwise no. That would wreck the game. In most cases, combat is going to be resolved well before 8 hours have elapsed anyway. This would be an interesting story twist in most cases I think.
I had a party of players who loved to fireball my encounters to death, so I made mindflayers that were trained to fight mages and could counterspell once a day
When you cast this on a tarasque
Nothing happens. Its immune to Charm effects.
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If a monster casts this on a player does it have any effect?
Despite the name saying monster, the effect text says creature, so I could see an argument for this applying to essentially almost anything that would get an Initiative if a combat began
Can a player use this spell to control someone who has polymorphed into a beast?
Yes, it works on any creature
Cast this on a Nightwalker.
do charmed creature know that it was charmed?
It doesnt need to be a beast; it can be any creature.
https://www.dndbeyond.com/sources/mm/introduction#SpecialTraits
No, because they have innate spellcasting. You can't upcast spells unless the stat block specifically says the spell is cast at a higher level. So a mind flayer cannot.
According to some content I've seen elsewhere, the default is that creatures who are charmed are only aware they've been charmed if the spell specifies they do. This high-level spell is strong enough that unlike charm person, the creature who's been charmed does not know after it wears off.
Mind Flayers should definitely be able to use this at 9th level. They are built to bring more brains to their colony.
Yeah, its been written somewhere that a charmed creature rationalises everything they thought and did during the charmed event as their own thoughts and volition. When the effect wears off, they wouldn't think "why'd i do that, maybe i was charmed" they'd say "i just... thought it was the right thing to do" when asked, and even when told they were charmed would say "naaah, that's nonsense, it was all me". Charming is basically gaslighting.
As for the dominate spells, the reason why its a wisdom save and not a charisma save is because you're not turning them into a puppet, you're changing their neural pathways that they believe that your commands are their own thoughts or that they think that your instructions are the most desirable for them to do, they're still performing of their own volition, they still want to do it, and when the spell ends, they'll continue to believe that it was all their ideas or that it was the best. They wouldn't go "nah, i would never do that" because you've magically gaslit them.
when it says "You can use your action to take total and precise control of the target." does that mean essentially possessing it