A Mage's Proposal is a a side quest available in the Underdark, which can be accessed in chapter 5 of Baldur's Gate II: Shadows of Amn and Baldur's Gate II: Enhanced Edition.
- Buy a Freedom scroll from the Duegar in the Underdark and cast it somewhere in the area to release the mage Vithal. The spell freedom works too, but you probably don't have a mage who has the level required to cast lvl 9 spells.
- Speak to Vithal and he will ask you to retrieve his Book of Rituals from the Svirfneblin.
- Speak to the innkeeper in the Svirfneblin village to the north and pay him 300 gold for the Book of Rituals.
- Return to Vithal with the book and follow him to the Earth Elemental portal to the southeast.
- Defeat the Greater Earth Elemental and then wait for Vithal to return from the Earth plane.
- Do the same for the Air and the Fire portals. He will reward you and you can demand a greater reward, but don't ask too much.