Once you've recruited Volo the Bard in Baldur's Gate 3, you can approach him and talk to him while you're in your camp. He'll tell you that he's been researching how to remove the Illithid Parasite that's squirming about in your head, and offers to try and remove it for you, but what happens if you actually accept his offer?
What do you get from Volo?
If you accept his offer then Volo will stab into your brain with a sharp pointy ice pick (it's a gruesome scene) and you'll start bleeding. He'll remove your eyeball, leaving you with only one eye, but will then pull out a second prosthetic eye that is apparently way better than your normal eyes but is also a completely different color. Volo will then run away from your camp and not come back though, so only take his offer if you don't want to see him ever again.
This eye, as it turns out is able to see any invisible creatures within nine meters of you and forces creatures to succeed in a Dexterity saving throw or completely lose their invisibility. While this won't see useful in the early portions of the game, once you hit Act 2 of the game you'll see the benefits of this just growing and growing.