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.