A Mage's Proposal - Baldur's Gate 2 Side Quest
A Mage's Proposal is a side quest that begins by speaking with Vithal in the Underdark. In order to make Vithal appear you'll first need to use a Freedom Scroll or spell somewhere in the zone; doing this will release Vithal from his Imprisonment spell and allow you to talk with him. You can find Vithal for the first time in the south-central portion of the zone. He's found at the intersection between the 3 elemental portals.
Once you've spoken to Vithal and begun this side quest he'll venture down to the Elemental Earth Portal and wait for you and your group here. Before meeting Vithal at this portal the first thing we'll need to do is visit the Svirfneblin Village and speak with the Innkeep, you're going to want to purchase the Book of Rituals that is sold here as it's required for the next part of the quest. You won't purchase this book from a vendor window but rather through dialogue with the NPC.
Note: If you're currently under the Drow illusion that Adalon gives you then you'll get the book for free.
When you're ready to proceed with this quest return to Vithal who is standing by the Elemental Earth Portal and give him the book. In order to complete this quest from here on out you're going to have to follow Vithal from portal to portal and defeat any elementals that come out of the portals.
Vithal will stop outside of each portal and wait for you to speak with him which will give you time to rest and save up. I recommend you do save before each portal because there is a chance that Vithal can be killed while fighting the Elemental Guardians; which will obviously fail this quest.
After completing the Air Elemental Portal it'll be time to divvy up the loot. Vithal will give you a Rod of Absorption with 1 charge left on it as your reward, you'll have the option to argue for more of the loot which you should absolutely do. If you pressure him for more you'll also get an Abi-Dalzim's Horrid Wilting Scroll as well as a Simulacrum Scroll.
You can pressure Vithal one last time which will result in you fighting him for the rest of the items. The other items he has on him are Incendiary Cloud, Time Stop, Protection from Petrification, Wail of the Banshee and the Skull of Death Helmet.
Important: Vithal will use Time Stop at the start of the fight and follow it up with an Imprisonment spell. If he uses Imprisonment against your main character it is going to be a guaranteed Game Over. For this reason I would recommend you either run your main character away before fighting Vithal or summon 5 pets to (hopefully) eat the Imprisonment for you.