Edwin's Companion Quest - Baldur's Gate 2
Edwin's Companion Quest starts the first time you enter the Graveyard District with him in your party. He will mention the Nether Scroll which is the item he is seeking for his quest, you'll also get a Journal entry for Find the Nether Scroll for Edwin. From the moment you enter the Graveyard District with Edwin and start this quest you'll be timed. If you wait too long to obtain the Nether Scroll Edwin will leave your party.
What you need to do for this quest is enter the Lower Tombs map then make your way to the southern portion of this map where you'll find the entrance to the Southern Tombs. Once inside the Southern Tombs our destination will be in the eastern section where you'll find a Lich named Nevaziah (pictured below). Despite Nevaziah being a Lich he isn't powerful like a Lich normally is - he should be a relatively easy target for you and your group.
Note: When Edwin picks up the scroll make sure to choose the dialogue option, "Will you be using that scroll to help us?". Apparently there are bugs that can happen if you talk about the scroll being a 'party treasure'.
The next update to Edwin's Companion Quest will come 5 days after obtaining the Nether Scroll. After 5 days pass you'll get some dialogue with Edwin where he deciphers part of the scroll and will receive 50k EXP, +20 Lore and -1 to all Saving Throws. You'll have to wait another few days after which Edwin will decipher yet another part of the scroll... This time he'll get transformed into a woman!
While Edwin is in his woman form he'll have a woman voice actor and there will be hundreds of new dialogue lines involving this change. For the most part everyone in your party teases Edwin about his sex change and he doesn't take it very well. I'd encourage a whole playthrough where you have Edwin transformed into a girl just to see all the different dialogue lines that there are.
Edwin will be stuck as a female for a few weeks ingame until eventually you're approached by Degardan, a Red Wizard looking for Edwin. You have 2 ways of resolving this when he approaches you. The first method is to reveal Edwin is now a woman which will cause Edwin to become hostile and attack you. The second method is to maintain Edwin's cover in which case Degardan will leave peacefully and return in a few days.
In a few days you'll be approached by Degardan again, this time he'll know that Edwin is now female and he'll dispel the illusion he has on him. Edwin will be happy the illusion is gone but will attack Degardan anyway. This won't be a difficult fight, once Degardan is defeated the quest will conclude.