If the target of Planar Binding fails its Charisma saving throw, it “must follow your instructions to the best of its ability”.
How does the target experience the “must” portion of the effect? Do they experience an utterly irresistible compulsion? Or does it seem completely natural to them to behave in the manner dictated by the instructions given to them?
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A bound creature must follow your instructions to the best of its ability. You might command the creature to accompany you on an adventure, to guard a location, or to deliver a message. The creature obeys the letter of your instructions, but if the creature is hostile to you, it strives to twist your words to achieve its own objectives. If the creature carries out your instructions completely before the spell ends, it travels to you to report this fact if you are on the same plane of existence. If you are on a different plane of existence, it returns to the place where you bound it and remains there until the spell ends.
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The bold text above seems to suggest that the spell does NOT make instructions “completely natural”, and that the creature is thus aware of when an instruction is at odds with its own preferred behavior. For example, if an evil Dao that prefers to torture their slaves is bound with this spell and instructed to “be kind, considerate, respectful, and loving to everyone you interact with”, that they are aware of both their preferences and the instruction and the dichotomy between them.
But this leaves unanswered what exactly their experience of the magical effect is, whereby they must follow the instructions. Do they try to act normally but find themselves unconsciously acting in accordance with the instructions (like Jim Carrey in “Liar, Liar”)? Or is their experience more akin to an ongoing internal struggle that they never win, in which they try desperately, every single time, to do what they want to, but are (somehow) forced to act in accordance with the instructions?
The Planar Binding spell is an abjuration, which feels problematic given that the primary effect of the spell is coercive rather than protective (enchantment would make more sense). Having it be adjuration makes it more powerful, since numerous entities are immune to enchantment but few are immune to abjuration. But I’m left unclear how the spell enforces the instructions and what the target’s subjective experience is. Knowing the answer to this question will help answer a whole flock of related questions.