Note: There's a timer for this quest.
This article is a Find the Nether Scroll for Edwin is a companion quest in Baldur's Gate II: Shadows of Amn.
Walkthrough
The quest is obtained by recruiting Edwin and travelling with him to Athkatla's Graveyard District. Edwin then mentions his desire to look for an ancient scroll in the southern dungeons of the lower tombs.
After making your way through heaps of undeads and spiders, the nether scroll is obtained from the tomb of an ancient lich. Edwin immediatly seizes it for himself.
A couple days later, Edwin deciphers a part of the scroll and gains 50,000 XP and +20 to his lore ability. Another couple days later he deciphers another part of the scroll and transforms himself into a woman without being able to turn back.
About a week or so after the Edwina incident, a red mage of the name Degardan presents himself to the party and asks information about Edwin. The party can then betray Edwin or keep his cover. Betraying him will cause Edwin to permanently leave the group.
A couple days after this first encounter, if Edwin still is with the group, Degardan will appear to the group and unveils Edwin's camouflage. Degardan then dispells the transformation that was affecting Edwin. To show his gratitude, Edwin then assaults Degardan.