Recover the Silver Dragon's Eggs - Baldur's Gate 2 Side Quest
Recovering the Silver Dragon's Eggs is a quest that begins in the Underdark during Chapter 5 by speaking with Adalon. When you accept this quest Adalon will cast Drow Illusions on you and your entire party which will allow you to freely travel around Ust Natha. Without this illusion the entirety of Ust Natha will be hostile to you and there will be nothing you can do inside of the city except fight.
Technically all you need to do in order to complete this quest is visit the Temple of Lolth in Ust Natha and steal Adalon's Eggs from the small chamber in the top of the area. A screenshot of the exact location you can find Adalon's Eggs is shown below. You will have to attack some Drow inside of the town to unlock the door leading to Adalon's Eggs; otherwise the lock will only be opened when you progress the main quest chain far enough. Despite the fact that you can easily grab these eggs at any time I would strongly recommend you complete the entire Ust Natha quest chain first for the most experience.
When you first enter Ust Natha under the effects of the Drow Illusion you will begin the Inside The Drow City side quest which is a long quest will various different steps and outcomes. Completion of that quest will begin the followup, The Drow Summoning Ritual, which is the quest that will give you access to Adalon's Eggs.
During The Drow Summoning Ritual quest you will have the choice to corrupt Adalon's eggs by allowing Ardulace or Phaere to give them to the demon. Alternatively, if you're playing a character with an Evil alignment you can trade Adalon's Eggs to the demon as well for various rewards. Only one of those rewards is worth your time though, check The Drow Summoning Ritual quest page for more information.
In order to complete this quest you will need to make sure Adalon's Eggs remain uncorrupted. This will require you to kill Ardulace and Phaere during the ritual or to swap out Adalon's Eggs with a pair of fake eggs that you get during The Drow Summoning Ritual quest. For more information about all the various branching paths and different outcomes check out The Drow Summoning Ritual quest page.
Once you've finished with Ust Natha and escaped return to Adalon and give her the eggs to complete this quest. You'll be rewarded with almost 80k EXP for every character in your party and Necaradan's Crossbow +3. Additionally Adalon will teleport you and your group to the Western Tunnels where you'll find the door leading out of the Underdark.