Do not take too long before getting hands on the Nether Scroll or Edwin leaves.
There're three entrances all leading to the lower tombs, enter from the pyramid in southwest part of the map, make your way through heaps of undeads and spiders then into the lower dungeons from south, the nether scroll is obtained from Nevaziah, who's become lich and can be found near his tomb in the eastern area.
When you pick up the scroll, do not choose "party
treasure" option lest you risk having bugs.
Five days later, Edwin deciphers a part of the scroll and will receive a permanent -1 bonus to all saving throws, 50,000 XP and +20 to his lore. Couple of days later another decipher of the scroll transforms himself into a woman without being able to turn back, from this point many companions will have very interesting (not funny to Edwina of course) conversations with HER.
About three weeks or so a red wizard of Thay by the name of Degardan presents himself to the party and asks information about Edwin, either betray him, which causes him to turn hostile and finish the quest here, or keep his under cover.
A few days after the first encounter Degardan will find you again and this time dispels the transformation and unveils Edwin's camouflage. To show his gratitude, Edwin then assaults Degardan.