The player does not know of the transformation but no problem with it either for the party or for. We played several APs, but second darkeness do not like. My players want to know the lore of the elves and I thought a good way would be that one of them becomes middle of the game.
According to AP explained in SC I think it really is evil elves in making this happen. The drow are the representation of the worst virtues of elves, petty, arrogant, envious, paranoid ... I do not think that this subject has a caotic evil character. After the transformation the alignment may change when they let the evil take over all their actions.
I think the interesting thing to change is to generate an evil feeling that grows inside the character. I think the Devils can play an important role, the lust for power is great character and combined with a strong manipulation of a powerful devil can take the character to commit horrible acts in order to have more power.
(I know that the drow worship the devil but I think this has to come later)
