Tadpole was then inserted in the host's cranium through a nostril, ear canal, or eye socket. It then burrowed into the brain and quickly devoured the gray matter, growing in the process as it replaced the brain with its own tissue and attached itself to the brain stem like a parasite. This initial process, which happened over a period of a few hours,[5] effectively completely replaced the victim's personality with the tadpole's, while still keeping the body alive for the second stage of the transformation.
It does both as you can see.
Source:
https://forgottenrealms.fandom.com/wiki/CeremorphosisBUT!
That is regular case, ours do neither ... thats why we are still oureself and in control. You could say that its just chillin inside our head.
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But i think Warlock argument is a valid one ...
Yes Warlocks who didnt trade their body and soul for power exists ... but that only mean that they arent who we are talking about.
And not just them ... how about Clerics of Evil Gods, like Bane, Bhaal, and Myrkul?
How about oathbreaker Paladins?
How about Necromancers, or whole Shadow weave ...
I mean this world offers amcountless opourtunities to questionable and potentialy corupting power ... so whats one more?
