Many of the early side quests you encounter in Baldur's Gate 3 will have you trying to find someone who can help you remove the Mind Flayer Tadpole that's in your head. Mind Flayers are from the Underdark, so it's reasonable to look for answers at the source of the problem.
In the Myconid Colony will be Blurg. After telling him about the Tadpole and your story, he'll introduce you to his friend Omeluum. Omeluum is a Mind Flayer who seems to be against their brethren and away from the control of the Elder Brain and wishes to investigate your unique situation. After explaining his plan to make a potion, he requests you to find the mysterious tower to gather the resources.
Finding The Mysterious Tower
The Mysterious Tower is the Arcane Tower located in the Underdark. To find it, travel to The Selunite Outpost. In the outpost, go out the broken window along the left side and climb down the rocks to the path ahead. Be prepared for a tough fight ahead, and proceed further down the path until you find the Arcane Turrets guarding the tower. The only way to damage these turrets is by using lightning magic.
After either destroying or running past the second and further turret, look to the large mushrooms that you can jump to along the left side of the tower. Jump along the mushrooms to get to the back end of the tower, with the final jump dealing a small amount of damage due to the distance. Now behind the tower, grab the glowing Sussur Bloom and pick the lock on the door to the tower. Add the Sussur Bloom to the engine in the center of the room to power the elevator.
Finding The Mushrooms
Take the elevator to the floor right below the main floor, with large mushrooms growing within. In the northwest corner of this room will be a bookshelf with multiple Timmask Spores you'll need to collect. After collecting those, head to the room's southeast corner to find the large mushrooms growing. The smaller plants around them are the Tongues of Madness, which also needs to be collected. After obtaining both items, return to Omeluum by fast traveling to the Glyph in the Myconid Colony.
Talking To Omeluum
After returning to Omeluum, he will mix the gathered ingredients into a potion, and you'll have the option to drink it. Drinking the potion will lead to a series of saving throws to fight the effects of the Tadpole. Whether you pass the saving throws or fail, the potion will fail to help and only strengthen the Tadpole. Feeling bad that his potion didn't help you. Omeluum mentions a ring he would give you but states it's too precious to hand over. You can either buy the ring or pass an intimidation check to get it for free. It's the Ring Of Mind-Shielding, which gives advantage on saving throws against being charmed.