As you explore the Underdark in Baldur's Gate 3, you'll eventually encounter the friendly Mind Flayer Omeluum at the Myconid Colony. He gives the quest Tell Omeluum About the Parasite which feeds into the Remove the Parasite main quest. He'll request that you collect two dangerous mushrooms from a nearby Arcane Tower in order to help him research your tadpole.
The Arcane Tower has some decent magical loot, and Omeluum rewards you with the Psionic Ring for your trouble, which suppresses your Illithid tadpole. With the ring on, you'll be unable to use your Mind Flayer powers actively in conversation, and it may have some play later in the story as things come to a head with your little passenger. The quest also has a lot of great story content, with a long sequence at the end featuring multiple skill checks that reveals more about your tadpole, and the tower itself is a stand-out set piece that's well-worth exploring.
