Coelacanth , 6 hours ago

As I recall there are two decisions. First you get the choice of the Astral Tadpole, which opens the outer ring of mindflayer powers and gives you black veins in your face and the status “half-Illithid”.

Later - as part of the ending - you can decide to become a full on mindflayer, because for some arbitrary narrative reason the mcguffin can only be operated by one.