There are no mechanical rules for that, so ask the DM. The only two things a Mindflayer can do according to the book that sound like that are casting Dominate Person, which is definitely temporary, and consuming your entire brain, which is both very permanent and very fatal. Any other method of mental enslavement is an invention or home rule of your DM, so you'll need to work out with them how it functions.
I ask because I was a cleric and got trapped into becoming a mindflayer’s thrall
There are no mechanical rules for that, so ask the DM. The only two things a Mindflayer can do according to the book that sound like that are casting Dominate Person, which is definitely temporary, and consuming your entire brain, which is both very permanent and very fatal. Any other method of mental enslavement is an invention or home rule of your DM, so you'll need to work out with them how it functions.
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Thank you! I can cast that! That helps a lot.