As we've previously detailed, the ending of Act 2 of Baldur's Gate 3 will task you with a few difficult decisions. One of these decisions will decide whether you can use some extremely powerful abilities, but will also come at the price of your humanity, so the question remains: should you consume the Parasite offered to you or not?
Should you accept the offer of The Emperor?
The Emperor, a more evolved Mind Flayer that has his own independence, will offer you a tadpole that unlocks the outer rim of Illithid abilities, including abilities such as flight, the ability to transform into a Displacer Beast without having to spec yourself into a Druid, the ability to use a free counterspell and a lot more. The downside is that you will change, gaining black veins across your entire body, and your companions will react very badly to you doing this. Your entire quest so far has been all about getting the tadpole out of your eye so that you can live a normal life, after all, so why would you do this?
You can choose not to take the parasite by performing a Wisdom check, but the Mind Flayer will give it to you so that you can use it later should you change your mind. You can consume this at any time, but be wary of the negative connotations it could have with your companions and with your romances.