Baldur's Gate 3 begins with players' hero(s) being captured by a Mindflayer and brought aboard an Illithid Nautiloid (A flying Organic Ship made of tentacles and nightmare fuel). This leads to them being infected with Illithid Tadpoles, horrible nightmare leeches that hide behind the victim's eye and begin the horrifying process known as "Ceremorphosis".
Baldur's Gate 3: All Illithid Abilities, Ranked & Explained
The Illithid Tadpoles play a vital role in Baldur's Gate 3's Story and also offer unique powers to the party during gameplay.